libmail-checkuser-perl 1.24-1 (s390x binary) in ubuntu focal

 Mail::CheckUser provides routines for quickly checking the validity of
 email addresses. It carries out the following checks:
 .
    * syntax of the email address
    * existence of MX or A records for the domain part (optional)
    * direct connection via SMTP simulating email delivery with MAIL
      and RCPT (optional)