Despite the fact that Email standards such as MIME don't specify any file size limits, but in practice email users will find that they can't send very large files(more than 20 or 25 MB).
Some experts say that "you can safely send files upto 10MB between any email providers without the fear of message not being delivered"
Also I hear that over the Internet a message will often pass through several mail transfer agents to reach the recipient. Each of these has to store the message before forwarding it on, and may therefore need to impose size limits.
According to above, I have three questions that have one direction:
- Why mail transfer agents need to impose size limits?
- is there any security reason for this limitation?
- Why the limitation has not been exceeded 25 MB until now?
Comment:"With MIME, a message and all its attachments are encapsulated in a single multipart message, with base64 encoding to convert binary into 7-bit ASCII - or on modern mail servers running Extended SMTP, optionally full 8-bit support via the 8BITMIME extension. the MIME encoding, which typically uses Base64 adds ~33% overhead"
Update: I think that the reason is not only because of mitigating DoS attack,because:
An attacker can not perform attack with one file! So he needs enough email transfers, if he have a mechanism for performing his attack with many email, and he is faced with file limitation,instead of sending 10000 email with 50MB, he performs attack with 20000 email and 25MB size.(as an example)