it is not in the SMTP RFC because it is a purely local matter that does not affect email routing. there are a number of servers that support it. wikipedia list

it is not in the SMTP RFC because it is a purely local matter that does not affect email routing. there are a number of servers that support it. wikipedia lists some if them: https://en.wikipedia.org/wiki/Email_address#Sub-addressing https://en.wikipedia.org/wiki/Email_address#Sub-addressing