This is an ISA configuration issue. You need to make sure that ISA is configured to allow port 25 (SMTP) from the internal network (Small Business Server included) to the Internet. ISA can be tricky to configure, and when it is integrated on the same server as the applications it is trying to protect, you are talking even more complexity. I would suggest, as a best practice, you use a separate server to set up ISA or simply use an inexpensive firewall device instead. I think you will have better luck.
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With Windows Small Business Server (SBS) 2000/2003, you also need to be a member of the BackOffice Internet Users security group or ISA Server won't let you browse the Internet or send email out. If the other systems are not having problems with email or browsing, only the laptop, then ISA Server is not going to be blocking port 25 outbound.
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This was first published in February 2005