The transfer protocol.
WHAT IT DOES
Transfers mail between servers, and accepts it from clients.
WHAT A TRANSACTION INVOLVES
Identifying the connecting server Declaring the sender Declaring each recipient Transferring the message Receiving acceptance or rejection
WHAT THE ENVELOPE IS
The sender and recipients declared during the transaction.
WHAT THE HEADERS ARE
The addresses inside the message.
WHY THAT DISTINCTION MATTERS ENORMOUSLY
They can differ, and authentication mechanisms check different ones.
WHAT RESPONSE CODES INDICATE
Success Temporary failure, to be retried Permanent failure, not to be retried
WHY TEMPORARY AND PERMANENT MATTER
Rejecting permanently when you should defer loses mail.
WHAT TO PREFER
Rejecting at the transaction, rather than accepting and generating a bounce later.
WHY
Accepting then bouncing sends mail to forged senders, making you a source of backscatter.
WHAT OPPORTUNISTIC ENCRYPTION MEANS
Encrypting if the other side supports it, otherwise proceeding unencrypted.
WHAT THAT IMPLIES
Mail in transit is not reliably confidential.
WHAT IMPROVES IT
Policies declaring that encryption is required for your domain.
