Bamboo.Mua uses SMTP relay wrong

[ruslandoga]

Originally posted by @thefeli73 in plausible/analytics#4342

Observed behavior:

1. plausible-conf.env defines SMTP_HOST_ADDR=mail.domain.com
2. plausible tries to send an email
3. bamboo looks up MX of mail.domain.com
4. MX of mail.domain.com may be something unrelated like feedback-smtp.eu-north-1.amazonses.com., for dmarc monitoring
5. bamboo looks up A record of feedback-smtp.eu-north-1.amazonses.com.
6. bamboo tries to establish SMTP connection to non-SMTP server.
7. {:error, %Mua.TransportError{reason: :timeout}}

Expected behavior:

1. plausible-conf.env defines SMTP_HOST_ADDR=mail.domain.com
2. plausible tries to send an email
3. bamboo looks up A of mail.domain.com
4. bamboo tries to establish SMTP connection to SMTP server which was specified.
5. profit.