Funny posting this here, since I was with SYMC till just a few months back... but here it goes.
Our anti-spam appliance is blocking several IP's of MessageLabs since it is sending spam to us, this client is sending out a lot of similar e-mails to none existing address on our side. Hence the anti-spam appliance on our side sees this as malicious behaviour and temporarily blocks your IP of that particular sending server. Since you of course use a lot of load balanced servers, we block a lot of them. Impacting other customers of this platform, which is a bad thing. And it impacts the reputation of a great hosted platform that you guys and gals have.
Just as a small part of the info
sending IP: 216.82.243.199 (reverse: mail1.bemta8.messagelabs.com)
sending IP: 216.82.243.197 (reverse: mail1.bemta8.messagelabs.com)
sending IP: 216.82.254.101 (reverse: mail1.bemta7.messagelabs.com)
to addresses that seem to be generated by code, at least the start of these are:
BudMsite023@[insert our domain]
BudMsite028@[insert our domain]
BudMsite095@[insert our domain]
Sender is claimed to be BudgetConfirmations@budgetgroup.com - but can be very well spoofed of course.