Outlook File Size Limit: Every Limit, With the Real Numbers
Search this and you’ll get 10 MB, 20 MB, 25 MB, 35 MB, 112 MB and 150 MB – all from confident-sounding sources. They’re all partly right, because “Outlook file size limit” describes four separate limits. Here’s every one, taken from Microsoft’s own service description, plus the encoding rule that explains why a 24 MB file fails a 25 MB limit.
For a Microsoft 365 mailbox, the default message size limit is 35 MB to send and 36 MB to receive — not the 25 MB you’ll see quoted everywhere. An admin can raise it anywhere from 1 MB up to 150 MB, but 150 MB only applies to mail that never leaves Microsoft’s datacentres; anything sent externally caps out at 112 MB because encoding inflates the message by about a third. Separately: your mailbox holds 50 GB or 100 GB depending on plan, and your local PST or OST file stops at 50 GB. On-premises Exchange Server defaults far lower, at 10 MB.
The reason every article gives you a different number isn’t that they’re careless. It’s that “Outlook file size limit” is four questions wearing one coat, and most guides answer whichever one they assumed you meant.
So this page separates them out, and every figure comes from Microsoft’s published Exchange Online service description rather than from another blog post. Start with the table below to work out which limit you’ve actually hit — the error message usually won’t tell you.
Which Limit Have You Actually Hit?
Four limits, four completely different fixes. Match your symptom to the row.
| What’s happening | The limit | Where to look |
|---|---|---|
| Outlook blocks you when attaching a file | Attachment size | Client-side check before sending |
| It sends, then bounces back with an NDR | Message size | Your server, or the recipient’s |
| “Your mailbox is full” or you can’t send at all | Mailbox storage | 50 GB or 100 GB by plan |
| Outlook won’t sync, or errors on the data file | PST/OST file size | 50 GB local file cap |
One distinction worth holding onto: an attachment limit is checked by Outlook on your machine before anything is sent, so you get stopped immediately. A message limit is enforced by a mail server, so the email leaves, travels, and bounces back minutes later. If you got an instant refusal it’s the first; if you got an NDR it’s the second.
Attachment and Message Size Limits: The Real Numbers
These come from Microsoft’s Exchange Online service description. The critical line, and the one almost nobody quotes correctly, is the default.
| Limit | Microsoft 365 / Exchange Online | Exchange Server (on-prem) |
|---|---|---|
| Default message size (send) | 35 MB | 10 MB |
| Default message size (receive) | 36 MB | 10 MB |
| Admin-configurable range | 1 MB – 150 MB | Set by your admin |
| Maximum, internal mail only | 150 MB | n/a |
| Maximum, mail leaving Microsoft | 112 MB | n/a |
| Message size, Outlook on the web | 112 MB | 10 MB |
| Message size, Outlook iOS & Android | 33 MB | 25 MB |
| Single attachment, Outlook desktop | 150 MB | 35 MB |
| Single attachment, Outlook on the web | 112 MB | 35 MB |
| OneDrive cloud attachment | 2 GB | n/a |
| Number of attachments per message | 250 | 1,024 |
| Encrypted message (Purview) | 100 MB | 100 MB |
| Encrypted message (legacy OME) | 25 MB | 25 MB |
| Subject line length | 255 characters | 255 characters |
Two rows deserve a second look. Outlook mobile is capped at 33 MB regardless of what your tenant allows, which is why a file that sends fine from your laptop fails from your phone. And on-premises Exchange defaults to 10 MB — dramatically lower than cloud, and the reason organisations that haven’t migrated hit this wall constantly.
If you send encrypted mail, note the 100 MB ceiling for Microsoft Purview Message Encryption. Our guide to encrypting email in Outlook covers which licences include it.
The 33% Rule: Why Your 24 MB File Won’t Send
This is the single most useful thing on this page, and it’s why the arithmetic never seems to work out.
Email can’t carry raw binary data. Attachments are converted into text using an encoding scheme, and that conversion makes them roughly a third larger. Microsoft states it directly: mail routed outside its datacentres is subject to an additional 33% translation encoding increase, which is exactly why the 150 MB internal maximum becomes 112 MB the moment a message leaves.
Outlook on the web builds this in defensively: it restricts you to 25% below whatever your configured limit is, precisely because it can’t know in advance whether your message will leave Microsoft’s network. Set a 100 MB tenant limit and OWA will refuse anything over 75 MB.
Mailbox Storage Limits by Plan
A different limit entirely — this is how much mail you can keep, not how much you can send at once.
| Plan | User mailbox | Archive mailbox |
|---|---|---|
| Microsoft 365 Business Basic | 50 GB | 50 GB |
| Microsoft 365 Business Standard | 50 GB | 50 GB |
| Microsoft 365 Business Premium | 50 GB | 1.5 TB auto-expanding |
| Microsoft 365 E3 / E5 | 100 GB | 1.5 TB auto-expanding |
| Office 365 E1 | 50 GB | 50 GB |
| Office 365 E3 / E5 | 100 GB | 1.5 TB auto-expanding |
| Office 365 F3 (frontline) | 2 GB | Add-on only |
| Exchange Online Plan 1 | 50 GB | 50 GB |
| Exchange Online Plan 2 | 100 GB | 1.5 TB auto-expanding |
| Shared mailbox (unlicensed) | 50 GB | Requires a licence |
You don’t hit the ceiling without warning. Exchange gives you three staged alerts, and knowing them tells you exactly how much runway you have left:
A useful detail if you’re near the limit: the Recoverable Items folder has its own separate 30 GB quota and doesn’t count against your mailbox. Emptying Deleted Items doesn’t reclaim mailbox space the way people expect — the items move there rather than vanishing.
PST and OST File Size Limits
If Outlook is throwing errors about your data file rather than about a message, this is the limit you’ve met. It applies to the local file on your computer, not to anything on the server.
| Outlook version / format | Default maximum |
|---|---|
| Outlook 2010 – 2024 and Microsoft 365 (Unicode) | 50 GB |
| Outlook 2007 and 2003 (Unicode) | 20 GB |
| Outlook 97–2002 (ANSI format) | 2 GB |
| Warning threshold (modern default) | 47.5 GB |
The 50 GB cap is set by two registry values, and there’s a trap in where they live:
- Close Outlook, then open regedit.
- Go to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST.
- Create a DWORD MaxLargeFileSize, decimal, in MB.
- Create a DWORD WarnLargeFileSize, at least 5% lower.
- For 100 GB: 102400 and 97280. Restart Outlook.
- The key says PST but governs OST files too — there’s no separate OST key.
- 16.0 covers Outlook 2016, 2019, 2021, 2024 and Microsoft 365.
- Values are in megabytes, not bytes, for Unicode files.
- Leave a 5% gap or Outlook can’t write its own metadata.
- Don’t go past 100 GB — performance degrades badly.
How to Send a File That’s Over the Limit
Ranked by how well each actually works, rather than by how often they’re suggested.
How Admins Raise the Limits
For anyone with Exchange admin rights. Changes take somewhere between fifteen minutes and a couple of hours to propagate to clients.
- Connect to Exchange Online PowerShell.
- Run Set-Mailbox with -MaxSendSize and -MaxReceiveSize.
- The ceiling is 150 MB either way.
- Or use the Exchange admin center under the mailbox’s mail flow settings.
- Existing mailboxes need Get-Mailbox piped into Set-Mailbox.
- New mailboxes inherit from the mailbox plan — update that too, or the change won’t stick for new starters.
- Check current values with Get-TransportConfig.
- Allow up to two hours before testing.
Two things worth deciding before you raise anything. First, the recipient’s limit still applies — a 150 MB tenant doesn’t help if the client you email caps at 20 MB, so this only solves internal problems reliably. Second, large attachments have real costs downstream in backup windows, journaling volume and eDiscovery exports. Most organisations land on a moderate increase paired with actively pushing people toward OneDrive links.
For the wider set of tenant settings around this, see our Microsoft 365 management guide.
Error Messages, Decoded
| What you see | What it means | What to do |
|---|---|---|
| “The attachment size exceeds the allowable limit” | Outlook blocked it before sending | Use a OneDrive link, or compress the file |
| 552 5.3.4 Message size exceeds fixed limit | A server rejected it — yours or theirs | Check whose limit is smaller; usually the recipient’s |
| “Your mailbox is full” | Mailbox storage quota reached | Empty Deleted Items, archive old mail, check Recoverable Items |
| “The file has reached its maximum size” | PST or OST at 50 GB | Archive, reduce cached mail, or raise the registry value |
| 552 5.3.4 Header size exceeds fixed maximum | Headers over 256 KB — usually a huge recipient list | Split the recipients across messages |
| Sends from desktop, fails from phone | Outlook mobile’s 33 MB cap | Send from desktop or web instead |
| Sends internally, bounces externally | The 33% encoding increase on outbound mail | Keep external messages under roughly 112 MB |
Frequently Asked Questions
For a Microsoft 365 mailbox the default is 35 MB to send and 36 MB to receive, and an administrator can set anything between 1 MB and 150 MB. The 150 MB maximum only applies to mail that stays inside Microsoft’s datacentres; anything sent externally is capped at 112 MB because encoding inflates the message by around a third. Outlook on the web tops out at 112 MB and Outlook mobile at 33 MB regardless of your tenant setting. On-premises Exchange Server defaults far lower, at 10 MB. And the recipient’s limit applies too — the smaller of the two always wins.
Because email can’t carry raw binary data. Attachments are encoded into text before sending, and that conversion makes them roughly 33% larger — Microsoft documents this directly as the reason its 150 MB internal maximum drops to 112 MB for external mail. Divide your limit by 1.33 for the real ceiling: a 25 MB limit takes about 18 MB of actual file. The limit also covers the entire message, so body text, inline images, quoted reply history and your signature all count toward it.
50 GB on Microsoft 365 Business Basic, Standard and Premium, Office 365 E1 and Exchange Online Plan 1. It rises to 100 GB on Microsoft 365 E3 and E5, Office 365 E3 and E5, and Exchange Online Plan 2. Frontline F3 licences get 2 GB. You’ll get a warning email at 49 GB, lose the ability to send at 49.5 GB, and stop receiving mail entirely at 50 GB, with senders getting a bounce. The equivalent thresholds on a 100 GB plan are 98, 99 and 100 GB.
50 GB by default in Outlook 2010 through 2024 and Microsoft 365, with a warning at around 47.5 GB. Older versions were lower: 20 GB in Outlook 2003 and 2007, and just 2 GB for the ancient ANSI format used up to Outlook 2002. The same 50 GB cap applies to OST files. You can raise it with the MaxLargeFileSize and WarnLargeFileSize registry values, but going much past 100 GB makes Outlook noticeably slower and more prone to data file corruption.
Close Outlook and open the registry at HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\PST. Create two DWORD values in decimal megabytes: MaxLargeFileSize for the absolute cap, and WarnLargeFileSize at least 5% below it — for 100 GB, that’s 102400 and 97280. Restart Outlook. Two things catch people out: the registry key is named PST but controls OST files as well, so there’s no separate OST key to find; and 16.0 covers Outlook 2016, 2019, 2021, 2024 and Microsoft 365. The 5% gap matters because Outlook needs room to write its own metadata.
Share a OneDrive or SharePoint link instead of attaching the file. Outlook offers this automatically when something is too big, and cloud attachments handle files up to 2 GB. It’s better than attaching in almost every way: the recipient always sees the current version, you can revoke access afterwards, and nothing clogs either mailbox. Zipping helps only if the content isn’t already compressed — a folder of Word documents shrinks a lot, a PDF or JPEG barely at all. Splitting across multiple messages works but pushes the reassembly onto the recipient.
Yes, up to 150 MB. In Exchange Online PowerShell, Set-Mailbox with -MaxSendSize and -MaxReceiveSize changes an individual mailbox; piping Get-Mailbox into it covers everyone at once. Newly created mailboxes inherit from the mailbox plan rather than from existing mailboxes, so update that too or the change won’t apply to new starters. Allow up to two hours to propagate. Bear in mind the recipient’s limit still applies, so raising yours only reliably fixes internal sending.
Yes. The limit applies to the whole message, not just files you attached — body text, formatting, inline images, quoted reply history and your signature all count. A signature carrying a logo, a headshot and a row of social icons is a real contributor, especially once it’s repeated down a long thread where every reply adds another copy. It’s rarely the sole reason a message fails, but it can be what tips a borderline one over. Image-free signatures avoid the problem entirely and also render correctly when recipients block images.
The Bottom Line
Work out which of the four limits you’ve hit before trying to fix anything — an instant refusal when attaching is a client-side attachment limit, while a bounce a few minutes later is a server-side message limit, and they need different solutions.
For most people the practical numbers are these: 35 MB default on Microsoft 365, about 26 MB of actual file once encoding is accounted for, 50 GB of mailbox, and 50 GB in the local data file. And when something genuinely won’t fit, a OneDrive link solves it better than any amount of compressing, splitting or raising limits.