Several e-mail virus scanners can be tricked into passing an EICAR
test file if the following conditions are met:

1. the EICAR file is encoded in Base64 including characters not in the
   standard alphabet (e.g. whitespaces) and
2. the part containing the EICAR file is nested within one or several
   levels of multipart/mixed content.

Details and PoC can be found at:
http://www.quantenblog.net/security/virus-scanner-bypass

Vulnerable products:
- BitDefender Mail Protection for SMB 2.0
- ClamAV 0.88.6
- F-Prot Antivirus for Linux x86 Mail Servers 4.6.6
- Kaspersky Anti-Virus for Linux Mail Server 5.5.10

Not recognizing the EICAR file, but aborting the scan:
- F-Secure Anti-Virus for Linux Gateways 4.65

Not vulnerable:
- avast! for Linux/Unix Servers 2.0.0