Error "Message Size Exceeds Limit" When Sending Large Attachments Print

What the error means

If you try to send an email with a large attachment and it fails with a message similar to:

552 5.3.4 Message size exceeds fixed maximum message size

or your mail program simply refuses with "attachment too large," the message is bigger than the maximum size a mail server is willing to accept. The message was not sent and will not be retried, you will need to reduce its size or share the file another way.

The limit that stopped you is not always ours. Every mail server in the chain applies its own maximum, so even when our server accepts your message, the recipient's server can still reject it for size. Gmail and Outlook.com, for example, cap incoming messages at 25 MB, and many company mail servers allow less, with 10 MB still common.

Our servers accept messages up to 50 MB, including the encoding overhead described below. In practice that means a file of around 35 MB is the largest you can reliably attach when sending through us, and the recipient's server still applies its own limit on top of that.

Why the actual limit is smaller than it looks

When a file is attached to an email it is encoded for transport, which inflates its size by roughly a third. A 20 MB file becomes an email of about 27 MB. So as a working rule, the largest file you can reliably attach is about 70 percent of the stated limit. If a server accepts 25 MB messages, treat 18 MB as your practical ceiling for the file itself.

How to fix it

1. For documents and images: compress before attaching. Zip the files, or export the PDF at a lower quality, or resize photos, a phone photo is often 5 MB or more and can usually be a fraction of that without visible loss for email purposes.
2. For anything genuinely large: send a link instead of the file. Upload the file to a sharing service such as Google Drive, Dropbox, or WeTransfer and email the link. This is the correct tool for large files, it is faster for you, kinder to the recipient's mailbox, and immune to size limits.
3. Do not split one file across multiple emails using archive-splitting tools unless the recipient has specifically asked for that. Many spam filters treat split archives with suspicion, and reassembly fails often enough to cause more support headaches than it solves.

If a sender cannot get a large file to you

The same logic applies in reverse. If someone tells you their attachment to you keeps bouncing for size, ask them to share it as a link instead. If receiving large attachments by email is a genuine and recurring business need for your account, open a support ticket and we can look at what is possible on the server side for your domain.

Good to know

Large attachments also count against your mailbox storage twice over, once in your Sent folder and once in the recipient's Inbox. If you find yourself regularly attaching big files, moving to link-based sharing will also keep you clear of the mailbox-full problems covered in our article on quota errors.