Best mail service to use for email activation etc?
Depence on your requirements.
Some use defferent mailservices (many suggested/recommanded here)
Some use paid mailservices
Other use self hosted solutions (free and paid are available for this)
You can use sendgrid, reachmail's easy SMTP, smtp2go, etc everything will work fine as long you have proper SMTP credentials to access SMTP service.
As mentioned, would depend on a few factors including your environment and use case.
I can personally recommend sendgrid though.
Which service is easy to setup and doesn't show your server IP in the headers?