He continuado investigando y os copio, por si os sirviera de ayuda, el codigo del logs:
PostCast Server 2.6.0
SMTP log started at 7/28/2007 11:58:16
Thread 1: 11:58:16 [<-->] : Connecting to gmail-smtp-in.l.google.com
Thread 1: 11:58:17 [<-->] : Connected.
Thread 1: 11:58:17 [--->] : 220 mx.google.com ESMTP b23si589947ugd
Thread 1: 11:58:17 [<---] : HELO 0.0.0.0
Thread 1: 11:58:17 [--->] : 250 mx.google.com at your service
Thread 1: 11:58:17 [<---] : RSET
Thread 1: 11:58:17 [--->] : 250 2.1.0 Flushed b23si589947ugd
Thread 1: 11:58:17 [<---] : MAIL FROM:<[email protected]>
Thread 1: 11:58:17 [--->] : 250 2.1.0 OK
Thread 1: 11:58:17 [<---] : RCPT TO:<[email protected]>
Thread 1: 11:58:21 [--->] : 250 2.1.5 OK
Thread 1: 11:58:21 [<---] : DATA
Thread 1: 11:58:21 [--->] : 354 Go ahead
Thread 1: 11:58:21 [<---] : .
Thread 1: 11:58:21 [--->] : 550-5.7.1 [83.33.63.139] The IP you're using to send email is not authorized
Thread 1: 11:58:21 [--->] : 550-5.7.1 to send email directly to our servers. Please use
Thread 1: 11:58:21 [--->] : 550 5.7.1 the SMTP relay at your service provider instead. b23si589947ugd
Thread 1: 11:58:21 [<---] : QUIT
Thread 1: 11:58:21 [--->] : 221 2.0.0 mx.google.com closing connection b23si589947ugd
Thread 1: 11:58:21 [<-->] : Disconnected.
Thread 1: 11:58:21 [<-->] : Error: 550 5.7.1 the SMTP relay at your service provider instead. b23si589947ugd
Total Messages: 1
Sent: 0
Bad: 1
Left: 0
Time: 6 s
Msg/h: 600
SMTP log closed at 7/28/2007 11:58:22
No me llevo muy bien con el ingles pero creo que viene a decir que el IP del usuario no esta autorizado
Si esto es asi, a que se puede deber?