Message Size Limits


Mailprotector enforces a "per message" size limit of 150 MB (both inbound and outbound). Messages over 150 MB will be rejected and returned to the sender.

Applies To:

CloudFilter, SafeSend, CloudMail, Exchange+, Office 365 (O365), Google G Suite

Determining Message Size

Message Size is not the Same as File Attachment Size

File attachment(s) in an email does not constitute a message size on its own. The protocol for email transfer encodes a message, including the header information, envelope data, content, and file attachment(s) which make up the entirety of the message size. A 120 MB file attachment may be below the message size limit, but the encoding can add overhead that pushes the message size beyond the limit.

Email Overhead Considerations

The email protocol is not very well equipped or designed for large file transfers. MIME encoding is extremely inefficient, often resulting in file sizes 30-40% larger than the original attachment. Also, the SMTP protocol was not designed and is not well suited for the lengthy transmission times needed for large files. SMTP can handle smaller file sizes and is convenient, but remember that SMTP can be transmitted in unencrypted, plain text.

Sender and Recipients Must Support Message Size Limit

Mailprotector supports 150 MB messages sizes, but that does not guarantee an email will be delivered. If a user sends an email from a server that only supports 50 MB messages, the user will receive a bounce back before it ever reaches Mailprotector's system.

On the receiving end, a sender may be able to send a 70 MB message from their server through Mailprotector, but the recipient's email security or server only accepts messages under 25 MB in size. The sender would receive a bounce back in this scenario as well.

For example, Microsoft's Office 365 supports 150 MB messages sizes while Google's Gmail supports 25 MB.

Large File Attachment Recommendations

Sending a file attached to an email is convenient and quick. However, file sizes continue to grow and sending larger file types such as pictures and videos is becoming more common. Mailprotector recommends using file-sharing tools such as OneDrive, Dropbox, and Google Drive for larger files. File-sharing tools are built for efficiently transferring files and provide more control, both regarding security and availability, than email's SMTP protocol.

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