Thunderbird problem

I can recieve mail but I cannot send it, when I try send mail I get this error:

An error occured while sending mail. The mail server responded: Relay access denied. Please verify your email address is correct in Your mail preferences and try again.

However if I send an ‘internal’ e-mail say from to it goes through no problem.

I’ve checked my settings and my e-mail addresses are ok, I’ve tried changing the port from 25 to 587 and changing my smtp server back to my ISP’s but neither of those made a difference.

I’ve only just changed over to dreamhost is it just me being impatient and my inof hasen’t been updated throughout the globe yet (although my website works fine on the dreamhost server already) or is it something else?

anyone got any ideas?

Have you enabled SMTP authentication?