Email gets pretty deep and there a various ways to structure your setup but best practice is a server for cold which you should expect to get burned and need multiple in time. A separate server for marketing blasts. A separate server or connection for general client communication.
If your app lets users connect emails/servers for each scenario and such this gets a bit more complex on how the connection happens.
Sending Health and platform/user relation with sub accounts vs keys vs direct oauth connection would also be important to consider.
Some setups requiring things like CASA certification.
If we add inbound as a requirement to this it gets more complex with how DNS works, setup types, avoiding MX overlap, user setup experience, etc
Over recent years it’s just gotten harder, in February google cracked down pretty hard on email delivery requirements.
I could easily write a novel here on all the scenarios, but to simplify it go back to my first paragraph for a very base answer to your question.
My bubble email plugin
1:1 paid consult booking link if you want a full breakdown and recommendation on what route to go for your use case.