Interworx supports SMTP AUTH on port 25, but tcp/25 is increasingly blocked by ISPs (and with good reason). The primary reason that tcp/25 is blocked is because only mail servers should be communicating with each other via that port. Mail clients (MUAs) who wish to send email should do so via the Submission port (tcp/587) on their Mail servers (MSAs and MTAs). There’s no reason that an Interworx server can’t act as an MSA on tcp/587 in addition to tcp/25.
While you’re at it, can you enable SMTPs submission on tcp/465?
appear to offer direction.