I've been happy with Mailgun for free, super low volume sites (i.e. single digit emails per month.) However, I've been shopping around for alternatives because

I've been happy with Mailgun for free, super low volume sites (i.e. single digit emails per month.) However, I've been shopping around for alternatives because I have a domain that won't pass their dashboard validation of DNS records. I checked DNS from a wide variety of other tools, and everything works. I waited weeks but the dashboard still reports problems, but it doesn't tell me _what_ the problem is, and their support, naturally, isn't the best at the free tier. So I couldn't get any help from them on what the problem was. I'm open to the idea that I misconfigured DNS, despite it matching other domains, and the DNS tools validating my work, but without someone on their end giving me debugging information, I haven't been able to solve my issue, and have to use an alternative.