Quick answer: On Exchange Server the default maximum message size is 10 MB, so any attachment close to 7–8 MB fails. To raise it you must change the limit at every level a message passes through: the organization, the Receive and Send connectors, the mailbox, and (for Outlook on the web) two web.config files. In Exchange Online (Microsoft 365) the default is 35 MB send / 36 MB receive and you can raise it to 150 MB per mailbox. Personal Outlook.com and Hotmail accounts are fixed at 20 MB and cannot be changed.
Default attachment and message size limits (2026)
| Platform / client | Default limit | Can you change it? |
|---|---|---|
| Exchange Server 2016 / 2019 / SE – organization (send and receive) | 10 MB | Yes – Set-TransportConfig |
| Exchange Server – Receive connectors | 36 MB | Yes – Set-ReceiveConnector |
| Exchange Server – Send connectors | 10 MB | Yes – Set-SendConnector |
| Exchange Server – Outlook on the web (OWA) | 35 MB | Yes – web.config |
| Exchange Server – Exchange ActiveSync (phones) | 10 MB | Yes – web.config |
| Exchange Server – Exchange Web Services (EWS) | 64 MB | Yes – web.config |
| Exchange Online (Microsoft 365) | 35 MB send / 36 MB receive | Yes, up to 150 MB |
| Outlook on the web in Microsoft 365 | 112 MB for a classic attachment | No (use OneDrive links, up to 2 GB) |
| Outlook desktop with POP/IMAP accounts | 20 MB | Yes – registry |
| Outlook.com / Hotmail personal accounts | 20 MB (message + attachments) | No |
The most restrictive limit always wins. If you raise the organization limit to 50 MB but leave a Send connector at 10 MB, outgoing mail over 10 MB still bounces.
Why a 25 MB file fails a 25 MB limit (the 33% rule)
Attachments are Base64-encoded before they travel, which makes them about 33% bigger. Microsoft's own example: a limit of 64 MB lets users send about 48 MB of real attachments. So set every limit roughly one third higher than the file size you want to allow:
- To allow 25 MB files, set limits to about 35 MB.
- To allow 50 MB files, set limits to about 67 MB.
- To allow 100 MB files (Exchange Online), set limits to about 135–150 MB.
Exchange Server 2016, 2019 and Subscription Edition (SE)
Run these commands in the Exchange Management Shell as an Exchange administrator. The example allows roughly 50 MB attachments (limit 67 MB). Change the number to suit your organization.
Step 1: Check your current limits
Get-TransportConfig | Format-List MaxSendSize,MaxReceiveSize
Get-ReceiveConnector | Format-Table Name,MaxMessageSize
Get-SendConnector | Format-Table Name,MaxMessageSize
Get-Mailbox -ResultSize Unlimited | Where {$_.MaxSendSize -ne "Unlimited"} | Format-Table Name,MaxSendSize,MaxReceiveSizeStep 2: Raise the organization limit
Set-TransportConfig -MaxSendSize 67MB -MaxReceiveSize 67MBIn the Exchange admin center (EAC) the same setting is under Mail flow > Receive connectors > More options (…) > Organization transport settings > Limits.
Step 3: Raise the Receive connector limits
Get-ReceiveConnector | Set-ReceiveConnector -MaxMessageSize 67MBStep 4: Raise the Send connector limits
This is the step most guides forget. The default Send connector limit is 10 MB, so large outgoing mail keeps bouncing until you change it.
Get-SendConnector | Set-SendConnector -MaxMessageSize 67MBStep 5: Check per-mailbox limits
Mailboxes use the organization limit unless someone set a custom value. To raise one user only, or to clear an old custom limit:
Set-Mailbox -Identity "user@example.com" -MaxSendSize 67MB -MaxReceiveSize 67MBStep 6: Raise the Outlook on the web (OWA) limit
OWA has its own 35 MB cap in IIS, so users see "the attachment is too large" in the browser even after Steps 2–5. Edit both files below on every Mailbox server (make a backup copy first):
%ExchangeInstallPath%FrontEnd\HttpProxy\owa\web.config%ExchangeInstallPath%ClientAccess\Owa\web.config
Change these values (the numbers shown are the defaults):
maxAllowedContentLength="35000000"– in bytes. For 67 MB use70254592.maxRequestLength="35000"– in kilobytes. For 67 MB use68608.- In the back-end file only: both
maxReceivedMessageSize="35000000"and bothmaxStringContentLength="35000000"entries (http and https bindings) – in bytes, use70254592.
Then restart IIS from an elevated command prompt:
net stop w3svc /y
net start w3svcStep 7 (optional): Exchange Web Services and mobile devices
EWS already allows 64 MB. Outlook for Mac and some third-party apps use EWS. If you need more, raise maxAllowedContentLength in FrontEnd\HttpProxy\ews\web.config and the maxReceivedMessageSize entries in ClientAccess\exchweb\ews\web.config the same way. Phones that use Exchange ActiveSync are capped at 10 MB by default in the ActiveSync web.config files.
Exchange 2010 and Exchange 2013
The same cmdlets (Set-TransportConfig, Set-ReceiveConnector, Set-SendConnector, Set-Mailbox) work on Exchange 2010 and 2013, and the default organization limit is also 10 MB. Both versions are long out of support. Exchange 2016 and 2019 reached end of support on 14 October 2025, and Microsoft's supported on-premises version is now Exchange Server Subscription Edition (SE), released on 1 July 2025. If you are still raising limits on 2010 or 2013, plan the migration first.
Exchange Online (Microsoft 365)
In Microsoft 365 you set the limit per mailbox. The maximum is 150 MB.
Option A: Exchange admin center
- Open the Exchange admin center and go to Recipients > Mailboxes.
- Select the mailbox, then open Mail flow settings > Message size restrictions.
- Set Sent messages maximum size and Received messages maximum size (up to 153600 KB = 150 MB) and save.
Option B: PowerShell for every mailbox
Connect-ExchangeOnline
# Existing mailboxes
Get-Mailbox -ResultSize Unlimited | Set-Mailbox -MaxSendSize 150MB -MaxReceiveSize 150MB
# Mailboxes created from now on
Get-MailboxPlan | Set-MailboxPlan -MaxSendSize 150MB -MaxReceiveSize 150MBEven at 150 MB, Microsoft limits each client: Outlook on the web allows about 112 MB for a classic attachment, because it keeps room for the 33% encoding overhead. For anything larger, attach the file from OneDrive.
Outlook desktop: "The file you're attaching is bigger than the server allows"
This message comes from Outlook itself. For POP3 and IMAP accounts (not Exchange or Microsoft 365), Outlook has a built-in 20 MB limit that you can change in the registry:
- Close Outlook and open Registry Editor (
regedit). - Go to
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences(16.0 = Outlook 2016, 2019, 2021 and Microsoft 365; use 15.0 for Outlook 2013). - Create a DWORD value named
MaximumAttachmentSize. - Set it in kilobytes, for example
51200for 50 MB. A value of0removes Outlook's limit.
Or run this in Command Prompt:
reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Preferences" /v MaximumAttachmentSize /t REG_DWORD /d 51200 /fThis only changes Outlook's own check. Your mail provider's server limit still applies, so a 50 MB message will still bounce if the server accepts only 25 MB. For Exchange and Microsoft 365 accounts, the registry value has no effect; use the server steps above.
Outlook.com and Hotmail: maximum attachment size
For personal Outlook.com and Hotmail accounts the limit is 20 MB for the whole message, and there is no setting to raise it. To send bigger files, upload them to OneDrive and choose Share as OneDrive link when attaching.
Common errors and what they mean
| Error | Where the limit is | Fix |
|---|---|---|
| "The file you're attaching is bigger than the server allows" | Outlook client (POP/IMAP) or mailbox send limit | Registry value above, or Set-Mailbox -MaxSendSize |
| Upload fails or spins in Outlook on the web | OWA web.config (35 MB default on Exchange Server) | Step 6 |
NDR with 5.3.4 "message size exceeds fixed maximum message size" | A connector or the organization limit | Steps 2–4 |
NDR with RESOLVER.RST.SendSizeLimit or RecipSizeLimit | Sender's or recipient's mailbox limit (Exchange Online) | Raise the mailbox limit, or the recipient must raise theirs |
Best practice: how high should you go?
- 25–50 MB of real attachment size covers almost every business need without delivery problems.
- Remember the other side: if the recipient's server accepts only 25 MB, your 100 MB email bounces no matter what you set.
- For large files, OneDrive or SharePoint links are faster and always get the latest version, and Outlook on the web allows linked files up to 2 GB.
- Write down every limit you change. The next Exchange cumulative update or reinstall can overwrite
web.configfiles, so check Step 6 again after updates.
FAQ
What is the default attachment size limit in Exchange 2016 and 2019?
10 MB for the organization and Send connectors, 36 MB for Receive connectors, and 35 MB for Outlook on the web. The lowest one that applies is the real limit.
What is the maximum attachment size in Exchange Online?
You can set up to 150 MB per mailbox. The default is 35 MB to send and 36 MB to receive. Outlook on the web allows about 112 MB for a classic attachment.
Why do users still get errors after I raised the limit?
Usually a Send connector is still at 10 MB, the OWA web.config files were not changed, a single mailbox has a custom limit, or the recipient's server is stricter than yours.
Can I increase the Hotmail or Outlook.com attachment limit?
No. Personal accounts are fixed at 20 MB. Share large files with a OneDrive link instead.
Does the registry change work for Microsoft 365 accounts?
No. MaximumAttachmentSize only affects POP and IMAP accounts in Outlook. Exchange and Microsoft 365 limits are set on the server.
Sources
- Microsoft Learn: Message size limits in Exchange Server
- Microsoft Learn: Configure client-specific message size limits
- Microsoft Learn: Exchange Online limits
- Microsoft Learn: Attachment size exceeds the allowable limit error
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