Irrelevant. The GP asked for a paid SMTP service. GP, FastMail is very good.
edoceo, the reason your flippant comment is irrelevant and unhelpful is that running an SMTP service means dealing with often intractable deliverability problems that are only surmountable by having your mail sent by a server that sends a large volume of legitimate email to build reputation as a sender.
It doesn't matter if you use postfix, exim, sendmail, BSD, Linux, or TempleOS, or manually wiggle magnets towards an open socket to send email, the reason the GP wants paid SMTP is so that he can rent access to a server with adequate deliverability to actually use his email.