[jlgaddis]

I've not heard that specific reasoning before but I can believe it -- I've seen MTAs continue to (attempt to) send mail to the "old" MX for several days or even weeks (in a few cases) after the RR (with my "standard" 2d TTL) was updated.

[toast0]

Microsoft Exchange used to cache MX lookups for the life of the process. Which is pretty frustrating, but I think that may have coincided with the ~ 45 day uptime cap, so it wasn't completely awful.