I'm getting quite a few NDRs from my Exchange 2003 server. Some come back "Status: 5.5.0" and some come back with "Status: 5.4.0"
I've done some research into these errors which seem to be caused by the recipients mail server. If it was only 1 or 2 then I'd be happy to put the problem down to the recipient's server but the frequency makes me think there's something else going on.
I have a suspicion what it might be but could do with some advice. 10 days ago the domain/mail server got black listed (due to a virus). That was swiftly dealt with and we've been clean since then, but do certain mail servers and spam filters remember that and keep our domain name listed as being a high chance of spam?
If our emails were getting rejected because the recipients mail server thought it was spam, is a Status: 5.4.0 / 5.5.0 the sort of NDR you'd expect to see?
Finally, if my suspicious is right - is there anything I can do improve our domain name credibility?