This is unlikely to be a portainer issue and more a config issue with that server in npm. Bit hard for us to troubleshoot without seeing the real error behind this. What does a browser like edge tell you? What does this one say say if you click details?
Agreed, I moved all mine from godaddy to cloudflare to save money. My email is with M365.
simple version:
save your existing mail locally
point the DNS record of your domain name at whatever new service you are buying (they should have instructions on that)
import some / all / none of the mail you saved locally to the new service as you see fit
now your email will come and go from your new mail service
I loved migrating to 3 nucs from a 2015 synology, so think you are 100% correct. (It allowed me to use TB networking for a 26Gbe ceph network)