I just wanted to thank everyone for a great quarter.  We had lots of different answers to this question.  Not so much on the DNS question but on the Exchange Server option.  This article explains it very well.  Notice that POP3 and IMAP are disabled by default.  SMTP is the normal protocol for client side.  So it looks like:

25 - Internet Mail Flow
80 - web based email unencrypted
443 - encrypted web based email
587 - SMTP traffic

https://docs.microsoft.com/en-us/exchange/plan-and-deploy/deployment-ref/network-ports?view=exchserver-2019