I host my business site through a VPS account. The instance runs Ubuntu and I'm using POSTFIX+Dovecot as my mail server.

For the most part, the mail server works fine. I have noticed, however, that I can not send mail from specific local networks. I noticed this at a client's office serval months ago. I can receive email, but any time I tried to send mail when connected to their network the connection would time out.

Since I could send my mail after leaving, I chalked it up to improper network configuration and didn't worry about it. Unfortunately I've recently moved, switched service providers, and am forced to use the service providers router due to the special set-up they put in place to give me DSL in the sticks--well beyond the typical range for a DSL run. Now I'm unable to send email from home, which is a problem.

I have tried sending email through my phone (using cellular service rather than my DSL) just to confirm the server is currently working. I'm not even sure where start debugging. Any ideas on how I might track down the issue would be greatly appreciated.