E-mailing is something that every company needs to do. We need to communicate with our customers in order to send notifications, bills, or registration e-mails.
In the companies where I've worked before, e-mailing always presented a problem such as e-mails not being delivered, or being delivered twice, with the wrong content to the wrong customer, and so on. It looks terrifying that something as simple as sending an e-mail could be this complicated to manage.
In general, e-mail communication is the first candidate to write a microservice. Think about it:
E-mail does one thing
E-mail does it well
E-mail keeps its own data
It is also a good example of how the Conway's law kicks into our systems without being noticed. We design our systems modeling the existing communication in our company as we are constrained by it.