3.2. Synchronization
Any email system that cannot guarantee successful delivery of a message has a significant deficiency. Section 6.1 of RFC 2821 explicitly states:
When the receiver-SMTP accepts a piece of email (by sending a “250 OK” message in response to DATA), it is accepting responsibility for delivering or relaying the message. It must take this responsibility seriously. It MUST NOT lose the message for frivolous reasons, such as because the host later crashes or because of a predictable resource shortage.
This is a significant statement, and it is an absolute requirement. Any email system that does not follow this guideline cannot claim to be compliant with the SMTP protocol. Essentially, this paragraph requires that once the message ...
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