Hi!

I just updated the first machine in our company to Mac OS X 10.8 Mountain Lion.
This client now has problems sending mail via Zimbra, receiving mail is not a problem.
Even worse, sometimes it seems to work!

Here are two snippets from the mail.log:
(domain names changed)

Here you can see the log, when sending mail does not work:
Code:
Aug  1 22:39:21 zimbra postfix/smtpd[8205]: connect from unknown[192.168.4.52]
Aug  1 22:39:21 zimbra postfix/smtpd[8205]: setting up TLS connection from unknown[192.168.4.52]
Aug  1 22:39:22 zimbra postfix/smtpd[8205]: Anonymous TLS connection established from unknown[192.168.4.52]: TLSv1 with cipher AES128-SHA (128/128 bits)
Aug  1 22:39:22 zimbra postfix/smtpd[8205]: 484E530302C: client=unknown[192.168.4.52], sasl_method=PLAIN, sasl_username=bbbbb@domain.com
Aug  1 22:39:53 zimbra postfix/smtpd[8205]: warning: network_biopair_interop: error reading 5 bytes from the network: Connection reset by peer
Aug  1 22:39:53 zimbra postfix/smtpd[8205]: lost connection after DATA (0 bytes) from unknown[192.168.4.52]
Aug  1 22:39:53 zimbra postfix/smtpd[8205]: disconnect from unknown[192.168.4.52]
And here are some log lines, only some minutes earlier, when it did work.
Same client, no config change, nothing different!
Code:
Aug  1 22:27:07 zimbra postfix/smtpd[2871]: connect from unknown[192.168.4.52]
Aug  1 22:27:07 zimbra postfix/smtpd[2871]: setting up TLS connection from unknown[192.168.4.52]
Aug  1 22:27:07 zimbra postfix/smtpd[2871]: Anonymous TLS connection established from unknown[192.168.4.52]: TLSv1 with cipher AES128-SHA (128/128 bits)
Aug  1 22:27:07 zimbra postfix/smtpd[2871]: C18A030302A: client=unknown[192.168.4.52], sasl_method=PLAIN, sasl_username=bbbbb@domain.com
Aug  1 22:27:07 zimbra postfix/cleanup[2875]: C18A030302A: message-id=<E07138C0-F757-45E7-8D5E-C8CE1320381F@domain.com>
Aug  1 22:27:07 zimbra postfix/qmgr[2982]: C18A030302A: from=<bbbbb@domain.com>, size=914, nrcpt=1 (queue active)
Aug  1 22:27:08 zimbra postfix/smtpd[2879]: connect from localhost[127.0.0.1]
Aug  1 22:27:08 zimbra postfix/smtpd[2879]: 1A0C0303031: client=localhost[127.0.0.1]
Aug  1 22:27:08 zimbra postfix/cleanup[2875]: 1A0C0303031: message-id=<E07138C0-F757-45E7-8D5E-C8CE1320381F@domain.com>
Aug  1 22:27:08 zimbra postfix/qmgr[2982]: 1A0C0303031: from=<bbbbb@domain.com>, size=1545, nrcpt=1 (queue active)
Aug  1 22:27:08 zimbra postfix/smtpd[2879]: disconnect from localhost[127.0.0.1]
Aug  1 22:27:08 zimbra postfix/smtp[2876]: C18A030302A: to=<aaaaaa@domain.com>, relay=127.0.0.1[127.0.0.1]:10024, delay=0.38, delays=0.15/0/0.01/0.21, dsn=2.0.0, status=sent (250 2.0.0 from MTAdomain.com([127.0.0.1]:10025): 250 2.0.0 Ok: queued as 1A0C0303031)
Aug  1 22:27:08 zimbra postfix/qmgr[2982]: C18A030302A: removed
Aug  1 22:27:08 zimbra postfix/lmtp[2880]: 1A0C0303031: to=<aaaaaa@domain.com>, relay=zimbra.domain.com[192.168.2.2]:7025, delay=0.06, delays=0/0.01/0/0.05, dsn=2.1.5, status=sent (250 2.1.5 Delivery OK)
Aug  1 22:27:08 zimbra postfix/qmgr[2982]: 1A0C0303031: removed
Did anybody else experienced this?
Any idea, what I could change in the config to make it running?
I tried several smtp-settings in Mail, but nothing helped!

Regards,
prahn