In other cloud providers, there is SMTP you can use, with API, or sub-user you made to login. In DigitalOcean you must to have 3rd provider for smtp.
In aws I was use free limit of less then 1000 in month and send SMTP without manage any mail server. I think DigitalOcean should provide this, with extra options (payed). But at least small package for free for who want to just start develop.