Emails with attachments are larger than expected

Products

• Email Security.cloud

Issue/Introduction

Emails sent through the Email Security.cloud infrastructure are larger than expected or emails with attachments under the email size limit are rejected as being too big.

Resolution

Email was originally intended solely for handling text-based messages. MIME encoding makes it possible to send binary files (non-ASCII text files) such as multimedia files, compressed archives, etc. over the Internet.

Almost all modern email clients support the MIME encoding and decoding process. The MIME process takes binary files and converts or encodes them to a plain text format for the trip across the Internet. The MIME process then decodes or reconverts the plain text back into the original binary format once the email reaches its destination.

The result of this process is overhead in terms of file sizes. In general, MIME encoded binary attachments are approximately 137% of the size of the original, pre-encoded file. For example, if you attach a 3MB .zip file to an email, the file will exceeds 4.1MB when it is in transit between servers.

 

When you set email size restrictions on your server or in the Email Security.cloud settings, the MIME encoding process should be taken into consideration.