"Are you aware that some of your emails are being blocked, due to oversensitive spam finders at the ISP end!"
AOL has, in the past, been a prime culprit for this - at some periods blocking ALL mail from some of the major Spanish ISPs, on the grounds of "excessive spam" from addresses with paricular ISPs.
However - they did at least respond to the initiator to say non-delivery was due to spam - it didn't juts get lost.
Some of the AS software that is in use is IMHO worse than a chocolate teapot - blocking legitimate emails wihtout so much as a "by-your-leave"
Complain LOUDLY and threaten with reporting to OffCom (or whoever) for interfering with the free passage of legitimate correspondence.