Here is the error from an Outlook 2003 client. The server that the Thunderbird client is sending to is an Exchange 2007 server w/SP1 - SCL:9X-TM-AS-Product-Ver: SMEX-8.0.0.1181-5.600.1016-16832.004X-TM-AS-Result: Yes-25.266300-8.000000-31X-TM-AS-User-Approved-Sender: NoX-TM-AS-User-Blocked-Sender: NoX-MS-Exchange-Organization-SCL: 9This does not happen with embedded URL's on Outlook clients. It is only happening with Thunderbird clients.Thanks!Roy

I don't really know the Thunderbird client, so I'm guessing here. It is a POP3/IMAP4client, right? Howare the Thunderbird clientssending SMTP mail to your server? Could you configure them to authentication when they send SMTP mail? Authenticated connections are always set to an SCL value of -1.Jim McBee - Blog - http://mostlyexchange.blogspot.com

Should be something about URLs management in Oultook. In the Exchange server there are filters enabling security on URLs.Probably Outlook manages URLs in a different way to bypass those filters OR the exchange is recognising Outlook clients and letting them pass.About this: how are you connecting to the exchange with Thunderbird? IMAP, POP, ... ???It could be that POP/IMAP clients are reputatedinsecure for sending mails with URLs. In that case you could only try to whitelist senders in the Antispam filters configuration.HTHDiego Castelli

Jim, yes they are authenticating directly to the Exchange server configured for outbound SMTP. So this is strange.