When running a send_mail command, the completion code coming back is U00020408 0000000000050007 or U00050007.
This is not currently listed in the documented return codes here - this article explains a bit more of what the code is.
Component : AUTOMATION ENGINE
The full message shoulde be:
The documentation will be updated to state:
"50007" - The SMTP server has rejected the E-mail recipient.
More information on this return code can be found in the "Messages" documentation: https://docs.automic.com/documentation/webhelp/english/AA/12.3/AE/12.3/Messages/Messages.htm#500xx.htm#50007