External accounts setting strange behavior

[a.mercurio]

I've installed the Open Source version on Zimbra and I was going to set an external account on zimbra:Preferences:External account

When I insert the login parameters for a mailbox ( that works on clients and on gmail importing) the system returns Error:Connection Reset

If i look on the mailbox.log i get some errors, i think from the testing process.

This appen only with certain mailboxes!

the domains that I used are hosted on two different machine in two different places and the only common point is that the two system uses Qmail for sending email.

The domains, and the related mailbox, works fine with external mail clients, webmail clients, gmail importing so I think in the script ( java?) there is a setting that I can modify for let the external account work.

Test that I've done:
Ping and Traceroute: the machine reaches the domain
Test sending a mail from the machine via ssh ( mailx -s test [mail address]
[message] ) and it works
Test the pop connection via telnet and it works

I've tried with the account of gmail (and the one of Telecom) and the test goes ok (on Zimbra) .

I need, if is possible, to know how Zimbra makes the test in the session so I can do in ssh shell and maybe solve this issue (probably I'll have to set some limit somewhere or in the handshaking process of Qmail)

Thanks in advance

[a.mercurio]

I've downgraded from 7 to 6.0.8

In this way the external mail works fine.

Now the question is: what's the difference in the two versions?

[a.mercurio]

When I upgrade from 6.0.6_GA to 6.0.12_GA the first problem comes up again.

Someone can say what is the fisical difference (changed files) between the two release?