Base64 encoding inflates every attachment by about 37 percent. Providers enforce their limit against the encoded size, so raw file sizes need to be smaller. Under 10MB is the safe universal target that clears every consumer service and most corporate gateways.
Compress under 10MB hits that target with iterative quality tuning.
Ten different attachment limits, from ten different providers, none of them exactly what the marketing pages promise. The good news is that the discrepancy follows one predictable rule, and once you understand the rule you can pick a target size that just works.
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, before you add MIME headers, transfer-encoding boundaries, and line breaks. The final wire size lands 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 headers, boundaries, and line breaks. What the server measures.
The provider's cap applies to that wire size. When Gmail says 25MB, it is measuring the encoded payload passing through its server. The raw file on your disk can only be about 18MB before its base64 version hits the ceiling.
Every provider, real limits
Here are the working ceilings for the major consumer and business mail services. "Advertised" is the number in the help article. "Working" is what actually gets delivered as a raw file after base64 overhead.
- Gmail (personal and Workspace)25MB → ~18MB raw
- Outlook.com free20MB → ~14MB raw
- iCloud Mail20MB → ~14MB raw
- Yahoo Mail25MB → ~18MB raw
- Proton Mail free25MB → ~18MB raw
- Proton Mail paid25MB → ~18MB raw
- Fastmail50MB → ~36MB raw
- Zoho Mail40MB → ~29MB raw
- Microsoft 365 business33MB → ~24MB raw
- Microsoft 365 enterprise E3/E5150MB → ~110MB raw
- Google Workspace Business Starter25MB → ~18MB raw
- Google Workspace enterprise25MB → ~18MB raw
- Corporate Exchange (self-hosted default)10-25MB advertised
- Corporate Exchange (real deliverability)~7-15MB raw
- Legacy corporate gateway~7MB raw
- Government / hospital / university~7-10MB raw
Two takeaways. Consumer services cluster around 18MB raw. Business services vary wildly, with enterprise Microsoft 365 the outlier at the top and legacy Exchange environments the surprise at the bottom.
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. Some servers bounce the failure back to you. Some silently drop it. Corporate environments especially tend to log the rejection and never notify anyone, which is why files sometimes disappear into apparent-success without ever arriving.
Pick a target for the recipient
If you know who is receiving the message and what mail infrastructure they use, you can aim precisely. If you do not, aim low and skip the guessing game.
Aim at their real ceiling
- Consumer Gmail or Yahoo: 18MB target
- iCloud Mail: 14MB target
- Microsoft 365 business: 20MB target
- Enterprise Microsoft 365: 100MB target
- Any corporate address: 10MB target
Under 10MB, always
- Clears legacy corporate gateways
- Clears mobile-first providers with tight caps
- Clears academic and government mail
- Survives forwards and relays
- No guessing, no bouncing
Three ways to fit under the ceiling
When the file will not compress far enough, you have three real options.
Compress the PDF
Size-targeted compression is the cleanest fix because it keeps the delivery experience identical. The attachment still arrives as an attachment. Nothing about the recipient's workflow changes. Our under-10MB compressor is tuned for the universal target.
Split the file
Two 8MB emails always beat one 20MB 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, unfamiliar UI for the recipient), but each bypasses attachment caps entirely. Best used for genuinely huge files or for organized recurring transfers with a known recipient.
The limits are not going up soon
Bandwidth is cheap, storage is cheap, and yet attachment caps have barely moved in a decade. The reason is that mail infrastructure was designed for small messages and the ecosystem is built around that assumption. Antivirus scanning, archival systems, indexing, spam classification: all of these get more expensive as attachment sizes grow.
Providers have concluded that the right long-term answer is cloud file sharing, and they are gradually making attachments harder to use as a way to nudge users toward Drive or OneDrive or Dropbox equivalents. That is why Gmail offers to upload to Drive automatically. That is why Outlook suggests OneDrive links. The attachment path is being deprecated slowly by product design, without ever being retired officially.
Practical implication: assume the ceilings above will not get more generous. Design your habits around the numbers you have.
Compress a PDF to the universal 10MB target
Clears every consumer service and almost every corporate gateway. Nothing uploaded.
Common questions
Details that come up while planning a big send.
01Why is the real attachment limit smaller than the advertised one?
Attachments get base64-encoded during transit, which inflates them by roughly 37 percent. Providers count the encoded payload against their cap, not the raw file, so a file that fits on disk may not fit on the wire.
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?
Corporate mail environments frequently cap inbound attachments at 10 to 15MB regardless of what the sender's provider allowed. Legacy Exchange servers, security appliances, and archival tools all contribute to the tighter ceiling.
04What is the safe universal target?
Under 10MB. It clears every consumer service and almost every corporate gateway. Compressing to that ceiling turns a variable success rate into a reliable one.
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 entirely in your browser using WebAssembly. Files are read into local memory and never sent to a server.