Your ISP is probably blocking SMTP-related ports inconsistently. Here's what I see from Comcast Residential:<p><pre><code> $ nmap -6 -Pn 2404:6800:4003:c02::6c
Starting Nmap 7.10 ( https://nmap.org ) at 2016-03-27 17:38 PDT
Nmap scan report for sc-in-x6c.1e100.net (2404:6800:4003:c02::6c)
Host is up (0.18s latency).
Not shown: 996 filtered ports
PORT STATE SERVICE
465/tcp open smtps
587/tcp open submission
993/tcp open imaps
995/tcp open pop3s
Nmap done: 1 IP address (1 host up) scanned in 13.51 seconds
$ nmap -Pn 74.125.68.108
Starting Nmap 7.10 ( https://nmap.org ) at 2016-03-27 17:38 PDT
Nmap scan report for sc-in-f108.1e100.net (74.125.68.108)
Host is up (0.19s latency).
Not shown: 996 filtered ports
PORT STATE SERVICE
465/tcp open smtps
587/tcp open submission
993/tcp open imaps
995/tcp open pop3s
Nmap done: 1 IP address (1 host up) scanned in 12.86 seconds</code></pre>