Hello
Login with SSH and check
telnet yourmainserver.com 25
and check Mail Server SSL
using: //email/testTo:
And then check the
systemctl status postfix
systemctl status dovecot
Check Snappy Mail Admin
I can't send mail - #6 by josephgodwinke.
Also try login with IMAP or POP3 to Outlook or any other mail clients. Use the proper credentials and try sending email from these external clients.