Cannot send mail

I had my work email set up on my iPhone 3GS and I upgraded to the iPhone 4 a few days ago, and now I cannot send mail from my work email account that I set up. I set it up exactly the same way as I had it set up on the old phone and I was able to send mail from that one, but I keep getting an error message now as well as several of my co-workers. It reads "Cannot Send Mail. A copy has been placed in your Outbox. The recipient "xxx@xxx.com" was rejected by the server." I have 3 other email accounts set up on this phone and they work just fine. I have tried changing the outgoing mail server port from 25 to 110 and 587, but I get this error when it's on anything except 25: "The SMTP server "xxx.xxx.com" is not responding. Check your network connection and that you entered the correct information in the "Outgoing Mail Server" field. I have double and triple checked the information I entered, and it is all correct. Can anyone offer some help?