One thing I got ahead of myself on was my mail.(domain) A Record.
I deleted it at the same time i changed the MX. While the MX record is the important one, the A Record pointed the mail.domain so they could get to the old email from outlook.
Pointed it back so they can (should be able to) get the last message in the old pop setup... includes a link to the new portal.
Things have gone okay thus far.