Can't connect to SMTP Server

As the topic states I have problems connecting to a SMTP Server. Many topic have ben created eith the same theme, but none of them I found helpful as most of them were created by minor typos.

Mail on OSX (I use 10.5.6) can't connect to the SMTP Server (POP works fine), Thunderbird on the same machine hasn't any problem (ruling out Firewall/Network Problems)

I tried using Authentifation with MD5 Challenge and just using a plain Password. Both produced similiar output when using Check Connection Dialog)

<pre>
CONNECTED Jan 25 16:26:56.558 [kCFStreamSocketSecurityLevelNone] -- host:smtp.domain.de -- port:25 -- socket:0x17d8f8e0 -- thread:0x1779ff10

READ Jan 25 16:26:56.692 [kCFStreamSocketSecurityLevelNone] -- host:smtp.domain.de -- port:25 -- socket:0x17d8f8e0 -- thread:0x1779ff10
220 id.hosting.info ESMTP

WROTE Jan 25 16:26:56.809 [kCFStreamSocketSecurityLevelNone] -- host:smtp.domain.de -- port:25 -- socket:0x17d8f8e0 -- thread:0x1779ff10
EHLO [192.168.1.128]

READ Jan 25 16:26:56.941 [kCFStreamSocketSecurityLevelNone] -- host:smtp.domain.de -- port:25 -- socket:0x17d8f8e0 -- thread:0x1779ff10
250-id.hosting.info
250-STARTTLS
250-PIPELINING
250 8BITMIME

WROTE Jan 25 16:26:57.092 [kCFStreamSocketSecurityLevelNone] -- host:smtp.domain.de -- port:25 -- socket:0x17d8f8e0 -- thread:0x1779ff10
QUIT
</pre>

Thunderbird also conects through Default Port 25 and it works. And yes I rechecked the password more than 10 times, I went so far and deleted all KeyChain Entries associated with that domain - to no avail.

The only idea I have left is that Mail can't cope with the "@"-Symbol in the username (as all usernames have the form username@domain.com).


Does anybody have clue?