Hi, We have two ISPs, and a single Postfix server behind our Linux firewall that gets NATed to in the same way from IPs on each of those ISPs' lines, with both of those given in the MX records. After getting complaints that some incoming mail was getting delayed by 5 or 10 minutes - sometimes much longer - I sent a bunch of test mail from elsewhere, and saw that my test emails all arrived either instantly or precisely 5 minutes late.
Turns out one of the incoming lines times out on the response, which gets logged on the sender but not the receiving system. Since most all mail is only 5 or 10 minutes late - assuming the sending system behaves well and goes to the next MX record in 5 or 10 minutes - it's not a particularly visible problem - except for some sending systems that don't behave well, which are out there. In telnetting to the two IPs, with the one that works the 220 line comes up right away. On the bad line, the "Connect to" and "Escape character is '^]'" lines come right up, but not the 220. If I then hit Enter the 220 line comes right up, but not without that. Turning off tcp_window_scaling on receiver, sender, or both doesn't change this. But the problem is specific to one ISP. Is this familiar to anyone? Thanks, Whit
