I'm setting up an SMTP server behind NAT. I'm a bit confused about the DNS requirements as I thought I could just setup the SMTP server and allow connections from the internal IP address on the machine and it would send out emails fine.
Now, I can get it sending emails to gmail accounts, but on other email accounts it seems to not be able to connect to their mail server.
I've read something about reverse DNS lookups - do I need to put in place some DNS records for a domain so the other mail servers can do a reverse lookup and check that I'm legit? Currently, the emails just come from the external IP address of the server.
The email sending problem has been identified as being non-standard line-endings in text only email. Make sure you use
"\r\n" instead of
"\n" in text emails as some mail servers will reject your messages otherwise!