The only way to track this is if you get a bounce back error. If there is no error, then it is up to the recipient to check their SPAM filter, Junk folder or Whitelist your domain. If it is not resolved, we can test their domain for configuration issues.
If you or your customer received a bounce back error, it is critical that you copy and paste that into a word document and attach it to this ticket.
Attach supporting documentation/files where applicable