Server :: Postfix Cannot Send Email From Thunderbird/Squirrelmail To Recipient Behind Firewall

My company email server has been working flawlessly for the last 5+ years. Recently, one of our clients put their email server behind a firewall (an assumption on my part; details below). Ever since, email sent from within the company (192.168.xxx.xxx internal address; all computers and server is behind a NAT firewall [Netgear FVX538]) is either being sent to the client after some delay (if email is sent using Outlook) or is being deferred until the messages expire (Thunderbird, Squirrelmail, etc.). Email sent to the client from anywhere outside the company (using Thunderbird, Outlook or any other email client) is also delivered without any problems (usually after a short delay).

All other emails to the World are being sent without any problems at all (both inside and outside the company; using any email client or webmail). I did contact the client's postmaster, but the client, being a large government agency, will probably not address the problem (if it is on their side) anytime soon. I am not sure if I can do anything from my end to solve the problem. Ever since the problem with the client began, I added two statements to the postfix configuration file (smtp_pix_workaround_delay_time = 20s and smtp_pix_workaround_threshold_time = 0s); this seemed to a bit - it reduced the delay for emails sent via Outlook to a few minutes (as opposed to 30+ minutes); emails sent using Thunderbird/webmail are still being deferred.

Server details: Dual quad-core processor machine, 32 GB RAM, dual 1 gbps network, running Fedora 14 (64-bit; loosely modeled along the lines of Perfect Server (with ISPConfig2) as described on the howtoforge website). Running Postfix (v. 2.7.1), Dovecot (v. 2.0.9).

Postfix details:

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
body_checks = regexp:/etc/postfix/mbl-body-deny
broken_sasl_auth_clients = yes
command_directory = /usr/sbin

[code]....

I emailed the client's postmaster suggesting they disable smtp fixup. But still, what bothers me now is that Outlook works (both within and outside the company) while Thunderbird, webmail, etc. (used by ~90% of the users) do not. In addition, all email clients from outside the company can send email (authenticated via the company server) to the client without any problems. Is there anything I can do on my end to make non-Outlook clients work within the company (webmail is hosted on the company server; so, it would have an internal address as well)?