a) You setup your autodiscover and SCP's to point to this
https://mail.domain.net/Do you have an internal DNS entry for mail.domain.net > pointing to IP Address of Exchange Server.
b) Whats your internal domain name and external domain name ? Are they same / different ?
c) Go here
www.testexchangeconnectivity.com/Test for inbound / outbound email.
After ISP change, did you change your IP address in MX records
Check yours here
www.mxtoolbox.comdid you ask your ISP to create a PTR record for your external FQDN to point to your public IP.
In your case, I think you will have to check with forefront team on that.
Please post back questions.