All our outgoing email messages get stuck in the Queue folder under mailroot, including the auto-generated Delivery Status Notification messages.
The application server (SharePoint) and the SMTP virtual server are running on the same machine, which is a local domain member. OS is Windows 2003 with IIS 6. Generation of email messages are triggered from within SharePoint web applications.
Running the SMTPDIAG , we cannot see anything wrong (please see the results below):
Searching for Exchange external DNS settings.
Computer name is WEB2.
Checking SOA for gmail.com.
Checking external DNS servers.
Checking internal DNS servers.
SOA serial number match: Passed.
Checking local domain records.
Checking MX records using TCP: xxxxx.com.
Checking MX records using UDP: xxxxx.com.
Both TCP and UDP queries succeeded. Local DNS test passed.
Checking remote domain records.
Checking MX records using TCP: gmail.com.
Checking MX records using UDP: gmail.com.
Both TCP and UDP queries succeeded. Remote DNS test passed.
Checking MX servers listed for email@example.com.
Connecting to gmail-smtp-in.l.google.com [22.214.171.124] on port 25.
Successfully connected to gmail-smtp-in.l.google.com.
Connecting to alt1.gmail-smtp-in.l.google.com [126.96.36.199] on port 25.
Successfully connected to alt1.gmail-smtp-in.l.google.com.
Connecting to alt2.gmail-smtp-in.l.google.com [188.8.131.52] on port 25.
Successfully connected to alt2.gmail-smtp-in.l.google.com.
Connecting to alt3.gmail-smtp-in.l.google.com [184.108.40.206] on port 25.
Successfully connected to alt3.gmail-smtp-in.l.google.com.
Connecting to alt4.gmail-smtp-in.l.google.com [220.127.116.11] on port 25.
Successfully connected to alt4.gmail-smtp-in.l.google.com.