Yeah I'm aware of possible issues that I will bump into when setting up XPEnology for business use, especially on the down time. I think for now I'll get an official Synology so I can have something up and running without much problems and will just use the XPEnology as a DIY project and see how well I can deal with it when issues arise.
I will need to research more on DKIM and SPF , but are those free DDNS enough to get around the static IP? How or to whom do I register my MX with? My understanding on the MX records is to be done on GCP or whoever I have my domain with, but correct me if I'm wrong.
Also, since you had tried their MailServer Plus, do you encounter any issue sending or receiving mails? Any mail relay service you can recommend to make it works for sending and receiving? And when someone said you might lost your mail when your machine lost power or connection, is that means it was sent successfully from the sender side but I failed to received it or the sender will be notified on a failed delivery?