Why your 24 MB attachment is rejected by a 25 MB limit

You check the file. It is 24 MB. The limit says 25 MB. You send it, and it comes back rejected.

This is not a rounding error and your email client is not broken. The file that leaves your computer is meaningfully larger than the file sitting on your disk, and almost nobody is told why.

Attachments are converted into text before they are sent

Email was designed to carry text. Not documents, not photographs, text. That decision was made decades ago and everything since has been built on top of it.

So when you attach a PDF, it cannot travel as a PDF. It is encoded into plain text characters that any mail server can safely pass along, and the encoding is not free. The most common scheme represents every three bytes of your file using four characters of text. Three becomes four, which is an increase of about a third.

That is the whole explanation. Your 24 MB attachment leaves as roughly 32 MB of encoded text, and a server enforcing a 25 MB maximum measures the message it receives, not the file you selected. The limit was never about the size of your file. It was always about the size of the message.

What to aim for instead

Work backwards from the stated limit rather than up to it.

Multiply the limit by roughly three quarters and treat that as your real ceiling. A 25 MB limit means aiming for about 18 MB. A 20 MB limit means about 15 MB. That leaves room for the encoding, for the message body, and for the headers that accumulate as the message passes between servers.

It is also worth knowing that two limits apply to every message and the smaller one decides. Your provider has to accept it for sending, and the recipient’s has to accept it for delivery. A large company enforcing a conservative maximum will bounce a message that your own provider was perfectly happy to send, which is why a file that worked for one colleague fails for another.

Why the number differs everywhere you look

Search for the limit and you will find several figures, all of them cited confidently. They disagree because they are measuring different things.

Consumer services publish a headline figure and generally substitute a cloud link above it, so the send appears to succeed. Business mail systems ship with a default that administrators then change, usually downward, to control storage and scanning costs. That is why the same attachment leaves your personal account without complaint and bounces off a client’s corporate server an hour later.

None of those published numbers is the one to aim at, because every one of them measures the encoded message rather than the file in your folder. This is also why the advice to check your file size in Explorer or Finder misleads people. That figure is real, and it is not the figure being tested.

If you want to know what actually went out, look in your sent folder rather than at the original. The message recorded there is the encoded one, and its size is the number the receiving server compared against its limit.

The things that do not help

Zipping it. This is the most common instinct and it is close to useless for the files people actually send. PDFs, JPEGs and PNGs are already compressed formats. Putting them in a ZIP recompresses data that has already had its redundancy removed, which typically saves a few per cent, and the encoding overhead then applies to the ZIP anyway. Zipping is useful for many small files or for uncompressed formats, not for a large scanned PDF.

Renaming or converting the format. Changing the extension changes nothing. Converting a PDF to a Word document usually makes it larger.

Sending it again. The limit is not a mood.

What does work

Reduce the images inside the document. Nearly all the weight in a large PDF is its embedded images, particularly if it came from a scanner, because a scan stores a photograph of every page rather than the text on it. Recompressing those images changes the number substantially while leaving the text as text, so the document stays selectable and searchable at the other end.

Split it. Two messages, each comfortably under the limit, arrive. One message slightly over it does not. For a long scanned document this is often faster than trying to compress it far enough, and the recipient can start reading the first part immediately.

Use a link deliberately rather than by accident. Some providers silently swap a large attachment for a cloud link. That fixes the size and introduces a permissions problem, because the recipient now needs access to your storage and the link may expire or be blocked by their security policy. If you are going to send a link, choose it, and check the recipient can actually open it.

The quick version

The limit measures the encoded message, not your file, and encoding adds about a third. Aim for three quarters of whatever the limit says. Compress the images inside the document rather than zipping the whole thing. If it still does not fit, split it rather than crushing it.

Our compress PDF and split PDF tools both run on your device, which matters here for a specific reason: the file you could not email because it was too sensitive to put in someone’s cloud storage should not be uploaded to a converter’s server on the way either.