E-mail Password problem

Originally Posted in General. Response recommended I try
here.

First, I do not know if this is the correct forum, but I
will post here and move or repost as necessary.

I have a small network comprised of 5 systems as follows.

1 windows 98 machine with Office XP installed
1 Mac 0S 9.2 machine
1 Mac OS 8.x machine
1 Linux file server for Mac/Windows file sharing only
(does not provide ANY other function on the network)

All are connected through a 16 port linksys 10/100 switch
with a Linksys single port router providing internet
access via DSL.

The entire system was working fine until 4 days after
installing the Linux Box(I think this is coincidental,
but including the info just in case) when we began having
trouble sending e-mails. We could share files, surf the
web and receive e-mail just fine from any machine. We
started getting error 553 at all machines when attempting
to send mail. Our web host told us to check our advanced
server settings for mail clients (Outlook and Outlook
express) to make sure that we had our accounts set to
authenticate for sending mail. This was set and working
before this and verified correct afterward.

The long and short of this is that after trying many
different things to find the problem we reinstalled OE on
the Macs and everything is working fine on them.
Reinstalling Outlook on the 98 box did nothing, it still
indicates the 553 error when trying to send e-mail. We
even connected a completely different computer, setting
up an OE account matching an account on the other
machines and were able to send and receive with no
problem, this seems to eliminate a problem with the
router or linux box.

After all of this we are convinced that we had some
strange event that coincidentally borked things on all
machines at once.

On the 98 machine, in addition to reinstalling Outlook,
Upgrading to SP3, installing Outlook Express, completely
removing all networking items (adapters, protocols, etc.)
and re-installing, checking for viruses, trojans, spyware
and the like, we even deleted the .pwl file and verified
that there was not a setting anywhere that was retaining
a password; we are still unable to get that machine to
connect to the mail server and authenticate.

If anyone has any idea where I can look for the possible
problem with this, I would appreciate it.

Thanks