Get an internet connection outside your network.
Type “cmd” in run, type “nslookup” in the cmd prompt, then type” set type=mx”
Now type in your domain name e.g example.com, press enter.
This will list your mx records.
Make sure it points to your SBS external IP,
It is quite possible that your ISP mx has the lowest preference(say 10, compared to yours which may be 11 or 20), if that is the case , then it means all e-mails will be delivered to your ISP’s mail server.
If you prefer to have all e-mails delivered to your ISP, then you can use the POP3 connector in SBS to pull your e-mails from your ISP. This option will ensure your mails never bounce back even though your server goes down.
The other option is to edit your dns records and give your SBS a lower preference number than your ISP’s mailserver. Then all mails come directly to your SBS.
Also, note that if smtp servers don’t find an mx record, e-mails are delivered to the A-record or the www record. So, if your ISP hosts your website, and www traffic goes there, then it’s quite possible, like I said earlier that all mails are being delivered to your ISP. Remember that it will be much easier to use POP3 connector then.
Let me know how it goes.