Why can't I send email using my hosting provider's mail server?

I lease space on a server to host my domain, which we'll call mydomain.com. The instructions from my hosting provider says my mail server address is mail.mydomain.com, smtp port is 25, pop3 is 110. It won't work. Tech support at my hosting provider says my ISP may be blocking port 25, preventing me from sending email, and that his servers are ok. I say that since I'm sending email using mail.mydomain.com, it has nothing to do with my ISP. My isp can block port 25 on mail.isp.com, but they can't block access on mail.mydomain.com, which is another server entirely. Am I wrong? I sent my ISP an email, but the answer was no help. She just sent a canned answer using keywords from my email, but it didn't answer the question at all.

Yes they can block you from anything! also try another mail client program.

Your ISP CAN block port 25. And most of them do. It filters all traffic TO or FROM port 25 on the network. My ISP blocks port 25, so I have to use workarounds. Ask your hosting provider if they have SSL SMTP running. If they do, they can tell you how to set it up. Most ISP's don't block SSL SMTP.

Otherwise, you can set up your mail account using your webhost as the POP server, and your ISP's mail server as the SMTP server. The people who get mail from you will never know the difference.