Some domains have backup MX-records at their ISP/IT-partner. In those cases, the SMTP-perimeter server setting is in place, but I've seen that some of the ISPs work with variable IP-ranges, ergo ME is not set correctly all the time (it does not support MX-records, but IP-addresses, and those pointers can vary in practice).
10 mail.domain.com, which is 1 IP
20 backup.isp.com, which might resolve now in 2 IP's, but tomorrow in 3, or 2 different IP's.
Not too common, but I know KPN is changing IP's really often.