If you cannot receive mail, check that the mailbox exists and that the domain DNS records point to the correct mail service. If you cannot send mail, check the SMTP server, port, encryption and mailbox password.
Also check the spam or junk folder, mailbox storage and the sender address. DNS changes may need time to update.
When contacting support, provide the sender domain, recipient domain, approximate time and the error message. Do not include passwords or private message contents.