Each provider publishes its own cap, from 20 MB for iCloud Mail to 35 MB by default for Microsoft 365. Attachments travel base64 encoded, about 37 percent larger than the file, and receiving servers such as Exchange count that encoded size. A file under 10 MB clears every consumer cap listed here even after encoding.
The exception is a self-hosted Exchange Server left at its 10 MB default. It counts the encoded message, so the file itself needs to be under about 7 MB.
A provider's help page gives one number, and it is only half the answer. Some servers count the file as it sits on your disk and some count the encoded message on the wire, and a message has to get past the sender's server and the recipient's.
The base64 tax
Email cannot carry binary data directly. The Simple Mail Transfer Protocol, dating from 1982, was designed for plain text and never quite outgrew that assumption. To send a PDF or an image or a spreadsheet through it, the file has to be re-encoded as text using a format called base64.
Base64 represents every 3 bytes of the original file as 4 characters of output. That is a fixed 33 percent expansion.
MIME then breaks the encoded text into lines of at most 76 characters, each followed by a two-character line break. Four thirds times 78 over 76 is 1.37, so the attachment on the wire is about 37 percent larger than the file on your disk.
The number you see in your file manager. What you assume the limit is checking against.
Encoded body added to the raw content. Fixed overhead from the encoding math.
Base64 plus MIME line breaks and headers. What a server that counts encoded size measures.
Where that increase counts depends on the server. Google's sending limits for Workspace accounts are measured before encoding, so Gmail will send a file close to its 25 MB cap.
Google's receiving limits are measured after encoding, and Google puts the increase at about 37 percent.
Exchange works the same way: Microsoft tells administrators to set a limit about 33 percent above the largest message they want to accept.
So a 22 MB file that leaves Gmail without complaint arrives as roughly 30 MB, and a server with a 25 MB receiving cap that counts the encoded message refuses it.
Each provider's published limit
These are the limits each provider publishes, linked to its own help page and checked on 28 September 2026. Providers change them, so the linked page is the authority.
- Gmail, personal25 MB to send
- Outlook.com25 MB per file
- iCloud Mail20 MB per message, in or out
- Yahoo Mail25 MB of attachments per message
- Proton Mail25 MB to send, 50 MB to receive
- Fastmail70 MB message, about 50 MB of attachment
- Zoho Mail, personal20 MB per message
- Microsoft 36535 MB to send, 36 MB to receive, by default
- Google Workspace, Business and Education25 MB to send, before encoding
- Google Workspace, Enterprise50 or 70 MB to receive, after encoding
- Exchange Server, self-hosted10 MB by default, after encoding
Most consumer services sit between 20 and 25 MB, with Fastmail the outlier at about 50 MB of attachment. Business mail depends on the administrator: Microsoft 365 can be raised as far as 150 MB, and a self-hosted Exchange Server left at its default stops at 10 MB of encoded message, which is about 7 MB of file. Outlook's own client limits are covered in what to do when a PDF is too large for Outlook.
Every server in the chain applies its own limit
The number that matters is the tightest one in the path your message takes. And the path always has more than one server.
Your outbound provider
Gmail, Outlook, whatever you send from. It applies the first check when you hit send.
Your organization's outbound gateway
If you are on corporate email, an outbound scanner between you and the internet often applies its own limit for data-loss prevention or antivirus reasons.
The recipient's inbound gateway
A security appliance in front of the recipient's mail server. Frequently the tightest limit in the chain and the least visible.
The recipient's mail server
The final destination. Applies its own per-user quota and message size cap.
If any one of these servers rejects the message, the delivery fails. A server that refuses an oversized message normally sends a bounce, a non-delivery report, back to you. A security gateway that quarantines or drops it may send nothing, which is how a large file can look sent and never arrive.
Pick a target for the recipient
If you know who is receiving the message and what mail system they use, divide their cap by 1.37. That is cautious for a provider that counts the file itself and exact for one that counts the encoded message, as Exchange does. If you do not know, aim low.
Their cap divided by 1.37
- Yahoo Mail, 25 MB: about 18 MB
- iCloud Mail or personal Zoho, 20 MB: about 14 MB
- Microsoft 365 at its 36 MB default: about 26 MB
- Exchange Server at its 10 MB default: about 7 MB
Under 10 MB, or 7 MB for old corporate mail
- Under 10 MB clears every consumer cap above after encoding
- It also clears Microsoft 365 at its default
- It is still too big for Exchange Server at its 10 MB default
- Under 7 MB clears that too
Three ways to fit under the ceiling
There are three ways to get a file through, and the first keeps the recipient's experience unchanged.
Compress the PDF
Size-targeted compression is the cleanest fix because the attachment still arrives as an attachment. PDFslime's Compress PDF under 10 MB steps image resolution and quality down until the file fits, and hands the original back untouched if it was already under. It measures the file, not the encoded message, so for an Exchange Server at its default compress to 5 MB instead.
Split the file
Two 8 MB emails beat one 20 MB email that bounces. Use Split PDF to break the source into halves or thirds, then send in sequence with clear part labels in the subject.
Send via link
Google Drive, OneDrive, Dropbox, WeTransfer. Each has tradeoffs (sign-in prompts, expiring URLs, an unfamiliar page for the recipient), but each bypasses attachment caps entirely. Use one for files far over any cap, or for recurring transfers to someone you know.
Why providers push links instead
Scanning, archiving and indexing all cost more as messages grow, and each provider's own answer to a large file is a link. Gmail swaps an attachment over 25 MB for a Google Drive link, Outlook.com attaches OneDrive files up to 2 GB, and iCloud Mail Drop holds attachments up to 5 GB for 30 days.
Compress a PDF under 10 MB
Clears every consumer cap listed here, even after encoding. Nothing uploaded.
Common questions
Details that come up while planning a big send.
01Why is the real attachment limit smaller than the advertised one?
Because the file is base64 encoded for the trip, which makes it about 37 percent larger, and receiving servers such as Exchange and Google's count that encoded size. Gmail's own 25 MB sending check counts the file before encoding, so a file can leave without complaint and still be refused on arrival.
02Which limit matters in a message's path?
Whichever server in the chain is strictest wins. Your provider's outbound limit, any organizational gateway, the recipient's inbound gateway, and the recipient's mail server all apply. The tightest one determines whether the message goes through.
03Why do corporate addresses reject smaller files?
Because the recipient's server sets its own cap, whatever the sender's provider allowed. The limit is a setting the recipient's IT team chose, so asking them for the figure is quicker than guessing, and a security gateway in front of the mail server can be stricter still.
04What is the safe universal target?
Under 10 MB for anyone on consumer mail or Microsoft 365, and under 7 MB if the address might sit on an older self-hosted Exchange Server. If compressing that far makes the pages hard to read, split the PDF or send a link instead.
05Do encrypted or signed messages take extra headroom?
Yes. S/MIME and PGP add signatures and encryption metadata that grow the encoded payload further. Aim for a lower raw file target when using either.
06Does PDFslime upload files during compression?
No. Compression runs in your browser with JavaScript: the file is read into the page's memory, rebuilt there, and never sent to a server.