The email attachment size limit isn't one number. It's the smallest number among every server the message passes through, and you don't control most of them. You attach a 30 MB video, your provider accepts it, and somewhere between you and the recipient a server you've never heard of decides 20 MB is the ceiling. Sometimes you get a bounce. Sometimes you get nothing at all, and you find out four days later when the client asks where the file is.
The workaround everyone reaches for is a personal file-sharing account, which means the file now lives with a third party under someone's individual login, with no expiry and no way to revoke it once it has been forwarded.
There's a better shape for this, and it doesn't require another product. Put the file in storage attached to the same account that runs your mail, and send a share link instead of a payload. A share link carries no size limit, costs one copy of the file instead of nine, and can be switched off after you've sent it.
Why Attachment Limits Exist and Why They Vary
SMTP has no size limit in the protocol. Every limit you've ever hit was a policy decision by an individual server operator, and the reason is queue economics: a mail server holds a message in memory and on disk while it works out where to deliver it, and a few hundred simultaneous 50 MB messages will exhaust either.
Common ceilings in the wild:
| Receiving system | Typical limit |
|---|---|
| Gmail and Google Workspace | 25 MB |
| Microsoft 365 and Outlook.com | Around 25 MB, tenant-configurable |
| Most shared hosting mail servers | 10–20 MB |
| Corporate gateways and appliances | Frequently 10 MB, sometimes lower |
The rule that governs your day is that the smallest limit on the path wins, and the path may include a gateway appliance in front of the recipient's server that neither you nor they can see. This is why "it worked when I sent it to my colleague" tells you nothing about whether it will work for a customer.
The worst outcome isn't the bounce. It's the silent drop: some gateways accept the message and then discard it, so your outbound queue reports success and the file simply never arrives.
The 33% Tax Nobody Mentions
Email bodies are 7-bit clean by inheritance, so binary attachments are Base64-encoded before transmission. Base64 encodes three bytes as four characters, which is a flat 33% expansion, plus line breaks.
A 20 MB file on your disk is about 27 MB on the wire. Against a 25 MB server limit, it bounces — and the error message quotes a size you don't recognise, because it's the encoded size, not the file size.
The practical rule: divide the published limit by 1.4 to get the real file size you can send. A "25 MB limit" is a 18 MB file. That number isn't a coincidence, and it's exactly where the automatic handover described below kicks in.
Attachments Are Also a Storage Problem
Send a 15 MB deck to eight colleagues on the same system and you haven't stored 15 MB. You have stored a copy in your Sent folder and a copy in each of eight inboxes: 135 MB of quota for one file, all of it identical, none of it deduplicated because each copy is embedded in a different message.
This is the reason mailboxes fill up. It's almost never the text. Run a search for larger:20M across all folders and the result is usually a short list of very large messages that account for most of the mailbox — several of which are copies of the same file sent to different people.
A link doesn't do that. One file exists once, and eight people hold a pointer to it.
The Share Link: a File That Leaves the Building
A share link is a public download URL for one file in your Drive. The recipient needs no account and no login. They get a plain page with the filename, the size, and a download button.
Three controls when you create one:
- Expiry. A date and time after which the share link stops working. Leave it empty for no expiry.
- Download limit. Anywhere from 1 to 100,000 successful downloads, then the share link closes itself. A limit of 1 is a genuinely useful setting for a document intended for one person.
- Note. Internal only, visible to you in the share link list. Six months later "Acme — signed SOW" is the difference between confidently revoking a share link and being afraid to touch it.
An expired or exhausted share link shows a short "no longer available" page and nothing else — not the filename, not who shared it, not when. That's deliberate. Someone holding a dead share link shouldn't learn anything from it.
Revocation is the property that attachments can never have. Once a file is inside a message it's gone: forwarded, archived, backed up, out of reach forever. A share link can be killed after it's sent, which matters the day someone forwards a contract to the wrong distribution list. Details in public share links.
The Automatic Handover at 18 MB
None of this helps if it requires you to remember it mid-compose. So the composer does the arithmetic itself.
Attach a file under 18 MB and it goes out as a normal MIME attachment, exactly as expected. Attach one over 18 MB and it's uploaded to Drive instead and inserted as a share link, with a 100-download limit already on it. You attach a file; the message decides how to carry it.
The limits for ordinary attachments are 20 files per message and 25 MB combined, with inline images counting toward both — a "small" message with fifteen screenshots pasted into the body isn't small. Send stays disabled while an upload is still running, so you can't dispatch a message whose attachment is only half uploaded.
You can also insert a share link to a file already in Drive without uploading anything again, which is the right move for the price list you send out twenty times a month. Update the file in Drive and everyone who follows the share link gets the current version. See large attachments via Drive.
Team Folders: Files That Stay Inside
A share link is for people outside the account. Inside it there's a better mechanism: shared folders.
The account owner marks a folder as shared and it appears in the Drive of every mailbox in the account, with a badge distinguishing it from personal folders. A mailbox owner can also create a top-level folder as shared from the start, which is how the person who actually curates "Brand assets" ends up owning it without routing the request through an administrator.
Sharing is binary: shared or not, with full read and write for everyone in the account. There's no read-only mode and no per-mailbox permission grid. That's a deliberate simplification — permission matrices are the part of file sharing that nobody maintains, and a half-maintained matrix is worse than a clear rule. If a file shouldn't be editable by everyone in the account, it doesn't belong in a shared folder. Details in sharing folders with your team.
Which One to Use
| Situation | Use |
|---|---|
| A 2 MB PDF invoice to a client | A normal attachment. It's fine. |
| A 400 MB video to a customer | Share link with an expiry date |
| A contract for one named person | Share link with a download limit of 1 |
| Brand assets the whole team needs | Shared folder |
| A price list you send constantly | A share link to the existing Drive file, so updates propagate |
| Nightly backups from a server | rclone to Drive with a scoped device password |
Practices Worth Adopting
Set an expiry by default. Not because a permanent share link is dangerous, but because one you forgot about is. Ninety days covers almost every real use, and a share link that outlives its purpose costs nothing to recreate.
Use the note field. An unlabelled list of thirty share links is a list you'll never clean up, because you can't tell which ones are still load-bearing.
Don't treat a share link as access control. A share link is unguessable, not secret. Anyone who has it can use it. For anything that must not leave a named recipient, set a download limit of 1 and a short expiry — and for anything genuinely confidential, use a shared folder inside the account instead, where access is tied to identity.
Sweep the large messages once. Search larger:20M across all folders, move what matters into Drive, and delete the messages. It's a fifteen-minute job that typically reclaims more quota than months of ordinary tidying — see mailbox search for the operators.
Remember that deleting is reversible. Files removed from Drive sit in Trash for 30 days. Emptying the trash is what actually frees space, and it's also the only step that's irreversible.
Put big things in storage first. If a file is going to be sent more than once, upload it to Drive and reuse one share link rather than re-attaching it. Mounting the drive over WebDAV makes that a drag-and-drop rather than a browser upload.
Frequently Asked Questions
What is the maximum email attachment size?
There's no protocol maximum — each server sets its own, and the smallest one on the delivery path decides. Gmail and Microsoft 365 both sit around 25 MB, corporate gateways are often lower. Because of Base64 encoding, a 25 MB limit means roughly an 18 MB file.
Why did my attachment bounce with a size error when the file was smaller than the limit?
Base64 encoding adds about 33%. The size in the error message is the encoded size on the wire, not the size on your disk.
Does the recipient need an account to open a share link?
No. They get a plain download page with the filename, size, and a button. No login, no sign-up.
Can I revoke a share link after sending it?
Yes, at any time, from the share-links list. The next person to open it gets a "no longer available" page. This is the main practical advantage over an attachment, which can never be recalled.
Does a share link count against my storage?
The file counts once, as a file in Drive. The link itself costs nothing, and creating ten links to the same file doesn't store it ten times.
What happens when a share link hits its download limit?
It stops working and shows the same generic unavailable page as an expired link. Create a new link if the file still needs to go out.
Is a shared folder the same as a shared mailbox?
No. A shared folder holds files and is visible to every mailbox in the account. A shared mailbox holds email and has explicit members with their own permissions. They are separate features that happen to share the word.
