Postfix locks 5.1-servers?

Hi,
are anyone familiar with conditions where postfix may bring a 5.1-p10
server to a halt, making the server accept incoming ports (such as 22) but
serve nothing, making getty(8) become non-respondent (pressing enter
doesn't give any feedback) and making the server ignore ctrl-alt-del etc?

I've got two boxes running FreeBSD 5.1-p10 and Postfix 2.0.10,1 and
2.0.13,1. They also have a java process (v1.3.1 and v1.4.1) running, but
that one is an extremely tiny process that can only be accessed from one
specific IP and when executing lives for a couple of milliseconds.

Does this ring any bells with anyone? We've been at it for a while and not
being able to figure it out. The boxes are quite different hardwareish, so
I don't suspect that being the problem.

Cheers

  Niklas Saers