: Port 587"> The real answer for most people is port 587. It turns out that this port has always been there and under the applicable standard is actually the preferred port:Most, and probably almost all mail server software supports authenticated submission on port 587. Any that dont are non-compliant and you should complain. Its essential that if you add port 587 support it enforces authentication; otherwise youre just trading off port 25 vulnerability for port 587 vulnerability. On the assumption that most of the situations wherein external access is a problem involve hosted domains, I asked several of the largest hosting services whether they support port 587 access for external users to hosted mail domains. Remember, just because their servers support it doesnt mean the hosting service opens the port on the firewall and enables it on the mail server. Here are the answers I got or didnt get ("yes" means they support port 587 for external users):
3.1. Submission Identification
Port 587 is reserved for email message submission as specified in this document. Messages received on this port are defined to be submissions. The protocol used is ESMTP [SMTP-MTA, ESMTP], with additional restrictions as specified here.
While most email clients and servers can be configured to use port 587 instead of 25, there are cases where this is not possible or convenient.
A site MAY choose to use port 25 for message submission, by designating some hosts to be MSAs and others to be MTAs.
- 1and1: yes
- Interland: yes (not just a fallback; their docs tell users to use 587)
- ThePlanet: "The Planets mail service does not currently advertise a submission service on TCP 587 by default, and weve never received a request to do so."
- EV1: No response
- Yahoo! Domains: No response (but I think the mail is managed by SBC, which supports port 587 on its own accounts
- GoDaddy: No response
- Verio: "As a business ISP, Verio does not block port 25 mail for its customers, but in many cases does enable alternate ports so that customers may continue using their servers for mail. The majority of server plans at Verio supports port 587some by default. Our dedicated hosting customers can enable or disable as they require."