SMTP RFC, recipient email address syntax

Question

Hi all !

Today I will ask you about SMTP RFC and specifically about recipient email address syntax.

Over the past few weeks, we issued an outage on our SMTP gateway which caused significant delays in outbound and inbound email delivery.
Investigating, we have found the root cause : on of our user has sent an email with a bad formatted recipient email address containing a bad character ("□" or something pretty similar).

Exchange has proceed this email flawlessly with no any error and has transferred this email to our SMTP gateway.
But this SMTP gateway did not like this recipient, which is understandable, and has fall into a loop of attempts to deliver this email, which caused an outage with delayed delivery.

My questions :

• About RFC : are special characters allowed in recipient email address ?
• If no, is it normal for Exchange to proceed email with bad formatted email address ?

Thank you !

Answer 1

Yes, you can use the allowed ones:

If Exchange allowed whatever "Malformed" SMTP address you are referring to, then Exchange wasnt enforcing it, but hard to know at this point