Similar problem, new thread wrong account when replying

This message pertains to email sent through a maillist. The problem
described is a minor annoyance. My interest is more to learn than to fix
the problem since this has me really puzzled.

Member A sends an email to the list using Outlook Express 6 and the default
email address.

Member B receives the message from the list and posts a reply.

Member A receives the reply from the list and clicks the reply button only
to discover that OE has chosen to use a non-default email address.

This happens to a few messages only. Earlier and later messages function
correctly. The problem messages remain a problem and do not correct with a
reboot/restart of OE.

Examination of the headers and comparison to the headers of other messages
do not reveal any clues that are recognizable to our limited knowledge of
OE.

All problem messages are not necessarily to-from-to the same person, but all
pass through the maillist.

It is unknown if each Member B uses OE 6.

Member A email accounts are with different ISPs (gmail and frontiernet).

Please suggest troubleshooting steps. Better yet, if possible, provide an
explanation of the problem and a solution. ;-))

Thanks,

Don