A search of the debian-backports mailing lists generates no hits for "pax", hope this isn't a known issue...
The version of 'pax' associated with squeeze appears to be broken for all .tar files:
clements@computer:/tmp$ cat> foo
oseutheth
clements@computer:/tmp$ tar cvf foo.tar foo
foo
clements@computer:/tmp$ pax -v< foo.tar
-rw-r--r-- 1 clements clements 4294967306 May 31 13:16 foo
pax: ustar vol 1, 1 files, 10240 bytes read, 13814518390877841716 bytes written.
... that is, it reports a size of (very large integer) for a tar file containing only one file of about 10 chars. This, in turn, means that the mail filter amavisd-new chokes on all .tar files that it receives.
The updated version of pax (1:20090728-2) does not exhibit this behavior on my machine (debian squeeze, i686), and it appears to me that adding the updated version to either squeeze-backports or to squeeeze itself would be a good idea.
Cheers!
John Clements