What is ESMTP SIZE clause?
How and why is an e-mail rejected because of its size?
The sending server can state the size of the message it is about to send. This way if there is a maximum account setting on the receiving server, it can refuse the e-mail and quit the current connection.
The main difference between this approach and the “account size constraints” in GMS Webmail is that ESMTP SIZE acts on the protocol level of e-mail transaction meaning that the message is not even be sent out so preserving network bandwidth.
It is worth noting that this isnt 100% efficient as mail servers are able to “lie” about the size of a message they wish to send.