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Attach & Send
Reference guide · reviewed 1 November 2026

Email Attachment Size Limits by Provider

Because SMTP encodes binary attachments into Base64 text, your file grows by roughly 33% in transit. A 20 MB file on disk can exceed a 25 MB email gateway limit.

+33%

Base64 MIME overhead on every attachment

25 MB

Most common gateway ceiling (Gmail, Yahoo, Proton)

~18 MB

Safe raw file size for external recipients

Email providerDocumented message limitSafe raw file sizeOperational notes
Gmail / Google Workspace25 MB (incoming & outgoing)~18.5 MBLarger files require external links; MIME Base64 encoding adds ~33% overhead.
Microsoft Outlook.com / Microsoft 36520 MB – 25 MB default (admin configurable up to 150 MB internal)~15 MB – 18 MBExternal recipients on standard gateways typically reject messages over 25 MB total.
Apple Mail / iCloud20 MB standard SMTP~14.8 MBMail Drop uses temporary links when enabled inside Apple Mail clients.
Proton Mail25 MB total message size~18 MBIncludes encrypted headers and attachments combined.
Yahoo Mail / AOL25 MB total message size~18.5 MBAttachments exceeding 25 MB after Base64 encoding fail to send.
Fastmail50 MB message size (recipient gateway dependent)~36 MB internal / ~18 MB externalEven if Fastmail accepts 50 MB, Gmail/Outlook recipients still enforce 25 MB.
FAQ

Why attachments bounce.

Email uses text-based SMTP and encodes binary attachments in Base64, converting every 3 bytes of binary data into 4 ASCII characters plus CRLF line breaks. That adds roughly 33% to 37% overhead, turning a 19MB file into a ~25.5MB message.

Upload the file to Attach & Send to create a private, expiring HTTPS download link (free up to 2GB with no account required), and paste the link into your email body.

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