Amavis seems to block some Word documents. I experienced those problems with signed documents. In the logfiles amavis writes:
DNS-Blacklist AHBL recently stopped their services. Since that I had many problems with our spamfilters. They are still using this blacklist and give all the mails a higher score because AHBL doesn't exist anymore.
The following entry in /etc/spamassassin/local.cf solvesthe problem(quick fix) :
score DNS_FROM_AHBL_RHSBL 0