Issue is on SelfReg/FEU but likely will appear in MAMS* also:

If a user registers with an invalid email address and the site is configured to
use SMTP to send email, an SMTP error will cause a fatal error when attempting
to push the user to MAMS/FEU.

Correcting the email address first allows the user to be pushed, but results in
a loss of all registration data as the username (email) no longer matches. Might
work better if changed in the db instead.

[12-Jul-2021 10:55:09 America/New_York] PHP Fatal error:  Uncaught
phpmailerException: SMTP Error: The following recipients failed: xxx@yyy.zzz:
The mail server could not deliver mail to xxx@yyy.zzz.  The
account or domain may not exist, they may be blacklisted, or missing the proper
dns entries.
 in /path/public_html/lib/phpmailer/class.phpmailer.php:1583