Email attachment size limits: Gmail, Outlook, Yahoo, iCloud and more

Gmail. Google's help page gives personal accounts a 25 MB attachment limit; if the attachments add up to more, Gmail removes them and adds a Google Drive link instead. For Google Workspace, the sending limit is 25 MB on Business, Education and Enterprise Standard, measured before encoding, with up to 50 MB on Enterprise Plus in Gmail on the web. The receiving limit is 50 MB per message (70 MB on Enterprise Plus), measured after encoding, and Google says it can't be raised. Gmail also blocks executable files such as .exe, even zipped.

Outlook.com and Microsoft 365. Microsoft's Outlook.com page gives a 25 MB attachment limit and up to 2 GB for files shared from OneDrive. Business mailboxes on Exchange Online default to 35 MB for sending and 36 MB for receiving, and an admin can set anything from 1 MB to 150 MB. Microsoft notes that mail leaving its datacenters grows by about 33% when it's encoded, so the 150 MB ceiling becomes 112 MB for mail to other providers.

Yahoo, iCloud and Zoho. Yahoo Mail caps the attachments in one message at 25 MB in total, incoming and outgoing. iCloud Mail takes messages up to 20 MB each way, or up to 5 GB with Mail Drop, which sends the files as links that expire after 30 days. Zoho Mail allows 20 MB per email on personal accounts and sets organization limits by plan, and it accepts incoming messages up to 40 MB.

People searching for the "Outlook attachment size limit" usually hit the desktop app's own cap. Microsoft's support page says the Outlook app limits internet accounts such as Outlook.com or Gmail to 20 MB per email, and Exchange (work) accounts to 10 MB by default, counting the message and its attachments together. That's a limit in the app, separate from what the mail service accepts.

The 10 MB figure matches the default on Exchange Server, the mail server many companies still run themselves: 10 MB per message received and sent until an admin changes it. It's one reason a large email can reach a Gmail address and bounce at a company domain.

Email was designed to carry text, so attachments travel as text too. Most clients and libraries encode them with Base64, which turns every 3 bytes of the file into 4 characters. RFC 2045, which defines it for email, says the encoded data is "about 33 percent larger" than the original. Encoded lines are also limited to 76 characters, each followed by a line break, which adds about 2.6% more. Together that's roughly 37%, the figure Google quotes for its receiving limit.

So whether a limit is measured before or after encoding matters. Google's 25 MB Workspace sending limit counts the files before encoding, while its 50 MB receiving limit counts the encoded message. Microsoft counts the encoded size too, which is why Outlook on the web only lets you send about 25% less than a mailbox's limit. HTTP email APIs carry attachments as Base64 inside the JSON body too, so a request with 10 MB of files is about 13.3 MB.

To go the other way, divide an encoded limit by 1.37: a 10 MB limit fits about 7.3 MB of files, and Microsoft 365's 35 MB default about 25 MB. Zipping rarely helps with PDFs, images and video, which are already compressed.

An oversized email can fail in two places. Your own provider or mail library can refuse it when you send, which you see straight away as an API error or SMTP reply. Or your provider accepts it and the recipient's server refuses it, which comes back later as a bounce.

Servers that support the SMTP SIZE extension (RFC 1870) advertise their limit in the EHLO reply. A sender can declare the message size in MAIL FROM, and the receiving server can refuse before the message is transferred:

A size bounce says nothing about the address: the mailbox exists and will take a smaller message. Don't retry the same email, which fails the same way. Send it again with the file as a link, and if your provider put the address on a suppression list because of the bounce, remove it first. For every other code, see the SMTP error reference and 550 permanent failure.

The SMTP relay sends through the same checks as the API, so a message from Nodemailer, PHPMailer or a framework mailer gets the same limits. Resending a stored email (POST /emails/{id}/resend) leaves the attachments out, because attachment bytes aren't stored.

Our limits are 10 MB per attachment and 20 MB per email, but the recipient's limit is what counts. A 10 MB PDF is about 13.7 MB once encoded, which a company mail server on a 10 MB default will refuse. For email that must reach anyone, such as invoices or exports sent to customers' work addresses, keep the whole email under 10 MB once encoded and link to anything larger.

Mailbox providers already work this way: Gmail turns oversized attachments into Drive links, Outlook.com offers OneDrive links and Apple Mail offers Mail Drop. An app can do the same, and a link has advantages beyond size: you can revoke it, see whether it was downloaded, and keep a sensitive file behind a sign-in.