Encoding Overhead with SMTP When emails are sent over SMTP, the encoding mechanism used to encode the email can make the total email size increase. When you have a 4 MB file for example attached to an email, encoding it in Base64 such that it is suitable to transmit over SMTP can incur a 33% overhead in message size. It takes approx 4 bytes to encode 3 bytes of real data. If you have set a limit on the server for the maximum size limit to be 5 MB, users may report that emails smaller than 5 MB are being rejected. This is expected behavior due to the overhead applied by the encoding method. This can also affect mailbox quota's in a similar fashion.