Mailgun and others also have a free tier for relaying smaller numbers of email. I've configured Postfix to use it for fallback if I can't deliver directly. I

Mailgun and others also have a free tier for relaying smaller numbers of email. I've configured Postfix to use it for fallback if I can't deliver directly. I use Digital Ocean and seem to struggle a bit delivering directly. 60 emails this month had to be delivered through mailgun since direct failed. Their free tier is up to 1250 emails per month.