zimbra html & thunderbird

[lk2oo3]

Hi all,

a friend tells me he is not able to direct click on link i send him via Zimbra web interface.
i was thinking the problem was his thunderbird setting about mail plain text only..
today i made some tests and discovered that he is right, but the reason why he does not receive my links is not the previous hypotesis.
Infact i configured thunderbird for a mail account on my pcs, configured to use html, and then wrote same email, with inside a link to firefox homepage ( http://www.firefox.org .. )
1. send email from non zimbra account 1 to tb account
2. send email from non zimbra account 2 to tb account
3. send email from my zimbra default email to tb account
4. send email from tb to zimbra account, resend from account to tb account

the fact is that only mail from zimbra email is not converted to a link (so i cannot direct click on http://www.firefox.org , and tb user does not receive the correct link to simply click on..)

but in case 4. (tb -> zimbra -> tb) the link is active.

I've search on tb and zimbra forums without success..
any idea on why this strange behaviour?

thanks in advance for any suggestion, Luca.

--
i tested 2 zimbra 6.05 FOSS installed on ubuntu 8.04 lts
thunderbird 1 client is ver 3.04 on winxp
thunderbird 2 client was icedove 2.0.0.24 on debian 5
my friend has a quite older ver of thunderbird on winxp