I am having a funny issue with sending to certain email domains, namely yahoo.co.uk. I had this problem awhile ago and it was to do with ESMTP inspect on my Cisco ASA which was fixed.
I have noticed I am getting event log 12014 & 12015 saying an internal transport certificate has expired. I am not sure as we don't use SSL how this applies to me.
Receiving system returned "Deferred" status 4.0.0, "Transient failure" is from the SMTP logs.
It also highlights the certificate expired as critical when I run Exchange BPA. I have found this article
but I dont know how it affects me as this is a single server environment.
Anyway the problem is intermittent and it seems to be happening after in ran
Exchange Server 2007 Update Rollup 6 for Exchange Server 2007 Service Pack 1 (KB959241) over the weekend.
Any thoughts would be appreicated