public Sending emails programmatically is straight forward. There are many libs and tools, that do this. E.g. there is swaks - the smtp swiss army knife - with which an email can simply be sent as follows (example is based specifically for mailgun):
swaks --to <recipient>@gmail.com \
--from git@homelab.lan \
--server smtp.mailgun.org:587 \
--auth LOGIN \
--auth-user 'postmaster@sandbox<sandbox-id>.mailgun.org' \
--auth-password '<password-by-mailgun>' \
--data "Subject: Test Email from Mailgun\n\nHello, this is a test email sent from the terminal."
It would be straight forward to simply implement email sending for each microservice of an application. But I don’t like the idea of distributing this.
Email Service
A much cleaner approach from my perspective is to apply an SMTP relay like Postfix - mail transfer agent which handles a low of things like queuing undelivered mails and resending them etc. It might also be a good idea implement the interface to Postfix on your own. E.g. you might want to implement an Email Service, that receives email requests via a message broker like Apache Active MQ Artemis with a self-defined message-payload format.