Devang,

The simple answer comes down to being a math issue.

The MIME standard introduced by RFC2045 in 1996 provided a framework to encapsulate a message and all its attachments in a single multipart message. This standard uses base64 encoding to convert binary into 7-bit ASCII. The base64 encoding adds up to an average of 37% overhead, though for very short messages the overhead can be a lot higher because of the overhead of the headers. Very roughly, the final size of base64-encoded binary data is equal to 1.37 times the original data size + 814 bytes (for headers). 

The sender you used for an example was sending a 21.5MB file which x 1024 = 22016KB. If you multiply that by 1.37 for the average base64 overhead you get 30,161.92KB which is greater than your setting Max Message Size setting of 29,999KB for the domain, and very close to your global Max Message Size of 32 in your SMTP settings, and in some situations could actually exceed the global Max Message Size if the email also had a large amount of text and hypertext markup, as seems to be happening in this particular instance.

If you are wanting to restrict emails up to a 30MB attachment then you would want to set the Max Message Size domain setting to 40750KB and your Max Message Size in your global SMTP settings to 41MB (as 29,999KB would only allow up to @ 20MB attachments).