Every email provider enforces its own attachment size cap, and most encode attachments in a way that adds 30%+ overhead. This reference table lists the current send and receive limits for major providers — and links to real sample files you can use to validate your app’s upload flow against each limit.
Send = max size you can compose and send · Receive = max size your inbox accepts. All sizes are after MIME encoding unless noted.
| Provider | Send limit | Receive limit | Test with |
|---|---|---|---|
Gmail Files larger than 25 MB are uploaded to Google Drive automatically and shared as a link. Gmail counts MIME-encoded size, so the practical limit on raw files is ~17–18 MB. | 25 MB | 50 MB | 10MB sample files → |
Outlook (Office 365) Default send limit is 20 MB for personal accounts and up to 150 MB on Microsoft 365, configurable by administrators. Exchange Server admins can also set custom limits. | 20 MB | 150 MB (admin-configurable) | 10MB sample files → |
Outlook.com (Hotmail) Larger files are offered as a OneDrive link. The hard cap on a single attachment is 20 MB, regardless of how many files are attached. | 20 MB | 20 MB | 5MB sample files → |
Yahoo Mail All attachments combined must stay under 25 MB. Yahoo offers cloud-link sharing for larger files via integrated services. | 25 MB | 25 MB | 10MB sample files → |
iCloud Mail Mail Drop kicks in for attachments over 20 MB and supports up to 5 GB, hosted for 30 days. Useful for sending video and large archives. | 20 MB | 20 MB | 10MB sample files → |
ProtonMail Up to 100 attachments per message. Encrypted attachments add overhead so plan for headroom under the 25 MB cap. | 25 MB | 25 MB | 10MB sample files → |
Zoho Mail Free plans cap at 20 MB. Paid Zoho Workplace plans support attachments up to 1 GB via Zoho WorkDrive integration. | 20 MB (free), 1 GB (paid) | 40 MB | 100MB sample files → |
AOL Mail Combined attachment size cannot exceed 25 MB. Older AOL Mail apps have lower limits — test on the modern web client. | 25 MB | 25 MB | 10MB sample files → |
Email attachments are MIME-encoded as base64, which inflates raw binary data by roughly 33%. A 20 MB file becomes ~27 MB on the wire. Most providers measure the encoded size, so the practical limit on a raw file you attach is well below the advertised number. Plan validation against the encoded size:
// Rough rule of thumb
const PROVIDER_LIMIT_MB = 25;
const SAFE_RAW_LIMIT_MB = Math.floor(PROVIDER_LIMIT_MB / 1.37);
// SAFE_RAW_LIMIT_MB === 18Don’t rely on a synthesized File blob in your tests — use real sample files at the boundary sizes:
Major providers integrate their own cloud storage — Gmail with Google Drive, Outlook with OneDrive, iCloud with Mail Drop — and replace the attachment with a share link. For your own apps, the standard pattern is the same: upload to S3/R2/GCS, generate a time-limited signed URL, and email the link instead of the file. Validate the cutoff client-side first to give users an actionable error before the upload starts.