Gmail over IPv6
An overseas network administrator contacted me to discuss the problem when conducting IPv6 email tests with Gmail. Understandably, some administrators think that Google Gmail can help to test IPv6 email setup. The fact is Gmail receives incoming emails from dual-stack mail servers based on the rule that v6 channel has priority over v4, but in sending out emails to dual-stack mail server, Gmail always selects the v4 path. I also doubt if Gmail can send out to IPv6 only mail servers. In the past, my IT colleagues thought our dual-stack mail server was wrongly configured after testing with Gmail and spent many hours of trouble-shooting with no clue of what happened. In the end, it was Gmail that used its own means of v4/v6 path selection without adhering to the dual-stack rule. I think this fact is now well-known to the IPv6 technical community.