What are the email attachment size limits, and why do big files bounce?

A file that attaches fine in your mail app can still bounce minutes later with "message too large". Attachment limits differ by provider, by edition and by app, and the number that counts is rarely the one on your screen. This guide lists the current limits, explains why encoding shrinks them, and shows admins how to change them.

What are the attachment size limits for each provider?

Most business providers cap a whole message at 25 MB to 35 MB by default. The limit covers the message text, inline images and all attachments together. These are the published figures at the time of writing (November 2026); check the linked vendor pages before you rely on them, because they change.

Sources: Exchange Online limits, Google's sending limits and receiving limits, Gmail Help, Microsoft's Outlook support page and Yahoo Help.

Two notes on the table. Google's receiving-limits page gives only two figures, "50 MB (Enterprise Standard) or 70 MB (Enterprise Plus)", while also saying its receiving limits "apply to all editions"; it doesn't name a figure for Business or Education, so check with Google if that matters to you. And Microsoft's Outlook support page still says the default for "Exchange accounts (business email)" is 10 MB; for Microsoft 365 mailboxes, the Exchange Online service description's 35 MB and 36 MB defaults are the current figures, and 10 MB is the Exchange Server on-premises default.

There are count limits too. Microsoft 365 allows 250 attachments per message, and Google says a Workspace mailbox can receive "up to 500 attachments per single email". Microsoft also caps total headers at 256 KB and rejects larger ones with "552 5.3.4 Header size exceeds fixed maximum size."

Why does an attachment under the limit still bounce?

An attachment under the limit can still bounce for two reasons: encoding makes it bigger, and every server on the path has its own limit.

Encoding adds about a third

Email was designed for text, so attachments are converted to text before they're sent. The usual method is Base64, defined in RFC 2045, which turns every 3 bytes into 4 characters. The RFC says encoded data are "consistently only about 33 percent larger than the unencoded data." Google gives a slightly higher figure for its receiving limit: its values "are the limit after encoding, which adds about a 37% increase."

Here's what that means for a 25 MB limit measured after encoding:

Providers don't all measure the same way. Google says its sending limits apply to "the total size of the message content and attachments before encoding", while its receiving limits apply after encoding. Microsoft says the opposite problem shows up at its edge: messages up to 150 MB work between its own users, but mail "routed outside of the Microsoft datacenters" is "subject to an additional 33% translation encoding increase, in which case the maximum message size is 112 MB." To stay safe, Outlook on the web restricts what you can send "to 25% less than the configured setting".

Microsoft's PowerShell documentation gives admins the same advice in reverse: "specify a value that's 33% larger than the actual maximum message size that you want to enforce. For example, the value 64 MB results in a maximum message size of approximately 48 MB."

The smallest limit on the path wins

A message passes through several servers: your client, your provider, perhaps an outbound gateway, the recipient's filter and finally the recipient's mailbox. Each one can refuse it. A 30 MB message that Microsoft 365 happily sends will bounce if the recipient uses personal Gmail or Yahoo, because they accept 25 MB.

Servers can announce their limit up front with the SIZE extension from RFC 1870. In the server's EHLO reply, SIZE 36700160 means it accepts messages up to 36,700,160 bytes (35 MB), and SIZE 0 means "no fixed maximum message size is in force". A sending server that sees the limit can refuse to send rather than waste the transfer. You can see a server's SIZE value yourself with the commands in how to test an SMTP server with Telnet and OpenSSL.

What does a "message too large" bounce say?

The bounce usually carries a 552 reply code. RFC 1870 says a message larger than the server's fixed maximum "will be rejected with a permanent failure (552) reply code", and that a server temporarily short of space replies with "452 insufficient system storage", which is worth retrying. Two common examples:

• Gmail: 552 5.3.4 Your message exceeded Google's message size limits, or a version that states "the size of your message (size bytes) exceeded Google's message size limits of limit bytes", from Google's SMTP errors reference. Gmail uses the same code for too many attachments and oversized headers.
• Generic: 552 message exceeds fixed maximum message size, the example wording in RFC 1870.

Our 552 5.3.4 reference page explains the variants, and a full mailbox, which also uses 552, is covered under 552 5.2.2. Read which server generated the bounce: if it's your own provider, the limit is yours to change; if it's the recipient's, only they can change it. The SMTP error code lookup decodes anything else.

How do admins change the size limit in Microsoft 365?

Microsoft 365 admins change the limit per mailbox; one documented way, shown here, is Exchange Online PowerShell. Microsoft Learn says the default MaxSendSize is 35 MB and the default MaxReceiveSize is 36 MB, and "a valid value is a number up to 150 MB."

1. Connect with Connect-ExchangeOnline.
2. Check the current values: Get-Mailbox -Identity alice@example.com | Format-List MaxSendSize,MaxReceiveSize
3. Change one mailbox: Set-Mailbox -Identity alice@example.com -MaxSendSize 50MB -MaxReceiveSize 50MB
4. Change every existing mailbox: Get-Mailbox -ResultSize Unlimited | Set-Mailbox -MaxSendSize 50MB -MaxReceiveSize 50MB
5. Change the default for new mailboxes with Set-MailboxPlan. Microsoft notes that "modifying the settings in a mailbox plan doesn't affect existing mailboxes", which is why step 4 is separate.

Remember that a bigger limit only helps on paths that allow it. Recipients outside Microsoft 365 keep their own limits, and Microsoft points out that "the size of message you can send or receive also depends on what your email client or solution supports."

How do admins change the size limit in Google Workspace?

In Google Workspace, only Enterprise Plus admins can raise the limit, and only for Gmail on the web. Google's admin help lists a Message size limit setting with this description: "(Enterprise Plus) Choose a message size limit between 25-50 MB in 5 MB increments." "The default limit is 25 MB."

1. In the Admin console, go to Apps > Google Workspace > Gmail > End User Access.
2. Select the organizational unit you want to change.
3. Find Message size limit and choose a value up to 50 MB.
4. Click Save.

Google's Workspace Updates blog announced the higher limit in February 2026, with a gradual rollout from February 23 and the incoming limit rising to 70 MB. Google adds a warning that matters if you route outbound mail through another service: "setting a limit higher than 25 MB can result in rejected messages" if your relay or outbound gateway has a lower limit. Check that service's limit first.

On other editions, Gmail handles oversized files for you. Gmail Help says that if attachments exceed the limit, "Gmail automatically removes the attachment and adds it as a Google Drive link in the email."

What is the safest way to send a large file?

The safest way to send a large file is a sharing link from your organization's own cloud storage. It avoids size limits completely and gives you control after sending. Microsoft says OneDrive file attachments in Outlook on the web "can be up to 2 GB", compared with 112 MB for classic attachments.

Use these options in order:

1. Share a link from OneDrive, SharePoint or Google Drive, limited to the named recipients, with an expiry date if your storage supports it.
2. Compress documents and images, or export a lighter PDF. Already-compressed files such as video won't shrink much.
3. Split the files across several messages if a link isn't an option.
4. Ask the recipient's IT team for a secure upload portal when they receive large files regularly.

Think about security at the same time. A password-protected ZIP gets past size limits but also past malware scanning; password-protected ZIP attachments explains why many filters hold them. Some file types are blocked whatever their size; see dangerous email attachment types. And because "a file has been shared with you" is a favorite lure, tell recipients to expect your link; file-sharing notification phishing shows what the fake versions look like.

How do you troubleshoot a "message too large" bounce?

Work through these steps before you change any setting. Most size bounces are solved at step 2 or step 3.

1. Read the bounce. Find the reply code and the name of the server that rejected the message. A 552 or 5.3.4 code confirms a size problem; 5.2.2 means a full mailbox instead.
2. Work out whose limit it is. If your own provider generated the bounce, the limit is on your side. If the bounce quotes the recipient's server, the recipient's limit applies, and raising yours won't help.
3. Compare the real message size. Add about a third to the total size of the attachments, then add the body and any inline images. Compare that figure with the limit, not the file sizes your computer shows.
4. Check the app. Outlook on the web, Outlook mobile and Gmail on the web each apply their own limits, which can be lower than the mailbox limit.
5. Check every hop you control, including any outbound gateway, relay or filtering service, because the smallest limit wins.
6. Resend as a link if the recipient's limit is the problem. That is faster than asking another organization to change its settings.

Does a spam filter or gateway change the limit?

Yes. Any filtering service or gateway in front of your mailboxes adds its own limit to the path. If the gateway accepts less than your mailbox, the gateway's limit is the real one. iBlockSpam's inbound filtering accepts messages up to 400 MB per message, so the effective limit for your users is still the one set by your mailbox provider. If you use an outbound gateway or relay, check its limit before you raise yours, for the reason Google gives above.

Next steps

• Look up your own mailbox limit in Exchange Online PowerShell or the Google Admin console, and write it down for your help desk.
• Tell staff the practical rule: attachments over about 15 MB should go as a sharing link, because many recipients accept only 25 MB.
• When someone reports a bounce, check which server sent it before you change anything. Paste the bounce headers into the Email Header Analyzer to see each hop.
• If a filtering service sits in front of your mail, confirm its size limit matches your mailbox settings; iBlockSpam's plans and limits are listed on the pricing page.