How to Send Large Files by Business Email (Limits and Alternatives)

You attach a 30 MB video, press send, and ten minutes later a bounce arrives saying the message is too large. Attachment limits are lower than most people think, and the receiver's limit matters as much as yours. This guide gives the real numbers, explains why files grow in transit, and shows the reliable ways to deliver something big.

Quick summary

• Gmail accepts messages up to 25 MB total. Outlook.com and Microsoft 365 default to around 20 to 25 MB depending on configuration. Many corporate servers cap at 10 MB.
• Email encodes attachments in base64, which adds about 33 percent. A 20 MB file becomes roughly 27 MB on the wire, so it fails a 25 MB limit even though it looked fine.
• The limit that bites is the smallest one along the path: yours, the receiver's, and any relay in between. You cannot know the receiver's limit, so plan for 10 MB.
• Rule of thumb: under 10 MB, attach it. Over 10 MB, send a link.
• Compress images before attaching. Photos straight from a phone are often 3 to 8 MB each; exporting at 1600 pixels wide brings them under 500 KB with no visible loss in email.

The limits you are working with

• Gmail accepts messages up to 25 MB total. Outlook.com and Microsoft 365 default to around 20 to 25 MB depending on configuration. Many corporate servers cap at 10 MB.
• Email encodes attachments in base64, which adds about 33 percent. A 20 MB file becomes roughly 27 MB on the wire, so it fails a 25 MB limit even though it looked fine.
• The limit that bites is the smallest one along the path: yours, the receiver's, and any relay in between. You cannot know the receiver's limit, so plan for 10 MB.
• Rule of thumb: under 10 MB, attach it. Over 10 MB, send a link.

Option 1: shrink it under the limit

1. Compress images before attaching. Photos straight from a phone are often 3 to 8 MB each; exporting at 1600 pixels wide brings them under 500 KB with no visible loss in email.
2. Save PDFs with "reduced size" or "optimise" in your PDF tool. Scanned documents shrink dramatically.
3. Zip a folder of files. Zip does not shrink already-compressed formats like JPEG, MP4 or DOCX much, but it turns twenty attachments into one.
4. For video, do not try. Even a short clip exceeds every limit. Use a link.

Option 2: send a link (the professional default)

1. Upload the file to your business file storage (Google Drive, OneDrive, Dropbox, or your own server) and create a sharing link. Set it to "anyone with the link" only if the file is not confidential; otherwise share with the recipient's address specifically.
2. Paste the link in the email with the file name and size, so the reader knows what to expect: "Proposal_v3.pdf (18 MB)".
3. Set an expiry on the link if the tool supports it, especially for quotes and contracts you will revise.
4. For one-off transfers to people outside your usual tools, a transfer service that generates an expiring link works well. Check the file is not sensitive before using a free public service.

Why links beat attachments even when the file fits

• A 9 MB attachment sent to five people costs 45 MB of storage across their mailboxes and yours. A link costs nothing.
• You can update the file behind the link; an attachment is frozen at send time.
• Large attachments trip spam filters more often and slow down phone sync for everyone in the thread.
• Your own mailbox quota lasts longer. OquMail gives 5 GB per mailbox free, but a habit of emailing 20 MB decks fills anything eventually.

Receiving large files

If customers need to send you big files, put a link to an upload folder on your website or in your signature rather than asking them to email it. Set your role mailbox (files@ or support@ on your domain) as the notification address so the team sees uploads. When a customer's attachment does bounce, the bounce message names the limit; reply with an upload link rather than asking them to try again.

Common questions

What does "552 message size exceeds fixed maximum" mean?

The receiving server rejected the message because it is larger than its configured maximum. It is a permanent error; resending will not help. Shrink the file or send a link.

Is it safe to send files by link?

As safe as the sharing setting you choose. A link restricted to the recipient's account is safer than an attachment, because you can revoke it. An "anyone with the link" URL is roughly as safe as an attachment: whoever has it can open it.

Can I send attachments through an email API?

Transactional email APIs are designed for notifications and receipts with short HTML bodies. OquMail's send API takes from, to, subject and html; for files, include a download link in the html rather than trying to embed the file.