Using Thunderbird and postfix on an intranet

I need to setup an email service on our intranet(internal LAN). I'm running rhel 7.0
I've installed Postfix 2.2.10.1-6.el7 and the Thunderbird Client (38.2.0)
I'm using IMAP with port 143 bound to the NIC card in my rhel 7.0 server that's "linked" to the intranet.
Same card (i.e. IP address ) is used by DNS (which works)
I've defined an IMAP server recvmail.ids4zos.net and an outbound smtp server sendmail.ids4zos.net.
The IMAP server has an MX record
nslookup and ping works for all nodes on the LAN.

I've defined the domain (ids4zos.net) and imap server host name (recvmail.ids4zos.net) to Postfix
All other parms are either commented out or set to their default value as installed.

I defined two "test" users to thunderbird
When I login as a test-user00 and try to send an email via thunderbird to the other test-user01.
Thunderbird keeps trying to write the message to test-user00's default sent folder, and never actually sends the message.

Any Ideas as to what I'm doing wrong ??

Best Regards

Guy