Why your attachments get rejected: the real limits of Gmail, Outlook and the rest

“This message exceeds the maximum allowed size.” Everyone has run into this refusal — often with a file that seemed perfectly reasonable. The attachment is one of email’s most used features, and one of its least understood: between the official limits, the encoding overhead and the rules of the server on the other side, here is what actually blocks your emails.

The official limits, service by service

Every email service sets a cap per message — attachments and text included. Here they are for the most widely used services:

Put into perspective, these limits belong to another era: a single photo from a modern DSLR weighs 8 to 12 MB, a minute of 4K smartphone video around 80 MB. Everyday files have grown a hundredfold in fifteen years; attachments haven’t moved.

The invisible overhead: encoding

Here is the trap behind the “inexplicable” rejections: an email does not carry your files as they are. Attachments are encoded as text (base64) to travel inside the message, and that encoding adds roughly a third to the real size.

A 20 MB file takes up about 27 MB once encoded — above Gmail’s 25 MB limit, even though the file looked like it would fit.

Rule of thumb: to stay under a 25 MB limit, your actual files should not exceed about 18 MB. That’s three DSLR photos. Not four.

The recipient’s server has the final say

Your email provider accepted the send? Nothing is settled yet. The limit that really matters is the receiving server’s — and it is often lower than yours, particularly in companies, where Exchange administrators frequently cut the cap down to 10 MB or even less.

The worst-case scenario is silent: some servers reject the message without any clear notification to the sender. You think the files went out; the recipient received nothing; nobody is told. It is one of the reasons a download link — which weighs only a few bytes in the message — is structurally more reliable than an attachment: the email always goes through, and download tracking confirms receipt.

The special case of ZIP archives

The classic reflex: zip it to squeeze it through. Two problems. First, the gain is usually marginal — photos, videos and PDFs are already compressed, so the archive barely shrinks anything. Second, and above all, email services distrust archives: security filters, quarantine, blocking of ZIPs containing executables. Your perfectly legitimate send can end up in quarantine without anyone being alerted.

We dedicated a full guide to the subject: sending a ZIP file without it getting blocked.

What to do when the attachment won’t go through

Three approaches, from quick fix to lasting solution:

1. Compress or resize — acceptable for light office documents, out of the question for photos, videos and scans where quality matters: the loss is permanent.
2. Cloud storage (Drive, OneDrive…) — effective within an organization, but it eats into your quota, often forces an account on the recipient and leaves permanent access rights to manage.
3. A dedicated transfer — the files leave as a download link: no attachment limit, no quarantine, no account on the recipient’s side. With TransferNow, up to 5 GB per transfer for free — 200 times Gmail’s limit — and up to 500 GB with an account.

For specific cases, our guides cover each situation in detail: getting past Gmail’s 25 MB, sending large files with Outlook, or sharing via WhatsApp without compression.