The limits, as they stand in 2026
Microsoft sets the figure most people hit first. Their support page states that for internet mail accounts such as Outlook.com or Gmail the message size limit is 20 MB, and that Exchange business accounts default to 10 MB. The limit covers the attachment and the message together, so it is the attachment size that has to clear it, not the finished message.
Around that: iCloud Mail stops at 20 MB, Yahoo and Proton Mail allow 25 MB, and WhatsApp sends pictures up to 16 MB before it starts recompressing them. Discord's free tier accepts 20 MiB per file. Portal and marketplace forms are the other common wall, and they sit much lower - a product photo on Etsy has to be 1 MB or less, while many government and school forms cap each file somewhere between 2 and 10 MB.
These numbers move. Two of the largest comparison tables published this year disagree with each other about Gmail, one saying 25 MB and one saying 20 MB, and Microsoft's own page says 20 MB for both Outlook.com and Gmail. Where a number matters to you, read it off the service rather than off a blog - the size limit is printed next to the upload box or in the provider's help.
Why a file that looks small enough still fails
The obvious cause is the limit itself: 21 MB into a 20 MB ceiling does not go, and there is no partial upload or warning that helps you understand it. But the less obvious cause is what happens between your file and the server.
Email transports attachments as text. A 19 MB PDF becomes roughly 25 MB on the wire once it is encoded, which is why a file that measures safely under the limit in your file manager can still be rejected by the receiving server. The other trap is rounding: a limit stated as 20 MB may be enforced as 20,971,520 bytes, and a file that lands a few kilobytes past it fails cleanly on the server even though it never looked close.
How close to aim
Treat the published number as a wall, not a target. For a 20 MB limit, compress to 18 MB or so; for 16 MB, aim at 14 MB. The gap absorbs container overhead and rounding, and it costs you nothing - at these sizes a little extra allowance usually means fewer pages get re-rendered and the document stays sharper.
Compressing is also not always the right answer. A 40 MB video does not belong in an attachment; it belongs in a link. For anything that is not a document, a shared link from Drive, OneDrive or Dropbox delivers the original quality, and the recipient gets a download rather than an inlined preview.
Where compression is right, it works best when you keep some slack. Our own targets are exact sizes rather than 'as small as possible', because a document that has been squeezed to its last kilobyte is often unreadable, and an unreadable attachment is worse than one that had to be a link.
What to check before you send
Confirm the limit on the service you are sending from - not the service you receive on, which may be stricter. Check whether your file manager and your mail client report the same size, since they count differently on some systems. Send a test copy to yourself first if the document matters and you have never used the target before.
And know what you are giving up. Compressing to fit a portal or a mail server is a format conversion, not a squeeze: pages that had to be re-rendered become images, so they stop being searchable and lose their links, form fields and signatures. Text that survives as text keeps all of that. If the recipient needs to fill something in, send the original.
One page per service
The three services people hit most often now have their own page, because each fails in a different way and the workaround is not the same: Gmail silently converts an oversized attachment into a Drive link, Outlook.com refuses the message outright, and iCloud Mail offers Mail Drop instead. Reading the one you actually send from saves a round of guessing.