I'm trying to get a mail server to support IMAP-over-SSL services. Everything is fine inside my network: Clients can specify SSL and port 993 in their email client configuration, and mail gets through. However, it's not working from outside: Somewhere, the port is getting blocked.
Nmap tells me this for the internal network:
[root@linux2 ~]# nmap -p993 192.168.0.4 Starting Nmap 5.21 ( http://nmap.org ) at 2011-04-18 09:54 PDT Nmap scan report for example.com (192.168.0.4) Host is up (0.00022s latency). PORT STATE SERVICE 993/tcp open imaps MAC Address: 00:1F:5B:35:F4:C8 (Apple)
And for the outside network:
[root@linux2 ~]# nmap -p993 xxx.xxx.xxx.xxx Starting Nmap 5.21 ( http://nmap.org ) at 2011-04-18 09:54 PDT Nmap scan report for example.com (xxx.xxx.xxx.xxx) Host is up (0.0023s latency). PORT STATE SERVICE 993/tcp filtered imaps
So, somebody is doing some filtering; the question is who. My router's firewall has 993 open; I've also experimented with (briefly) disabling the router firewall altogether, but with no improvement. I've checked with my service provider (AT&T DSL), but they say they're not blocking it.