Hi everyone, I'm pretty sure I've narrowed this down to being a Dreamhost problem, rather than a Thunderbird problem - any help would be fantastic!
I installed TB for a friend, who has a DH account, but despite all the details being right, they couldn't send or receive emails. On the TB forum over at Mozilla, it was suggested that if I didn't need to use secure authentication to uncheck the "use name and password" in the outgoing mail settings. I didn't know whether I did or not, so I unchecked it. Everything worked okay for a while, but now I'm getting "relay access denied" messages - however these are only when I try to email non-dreamhost accounts such as yahoo or gmail. When I tried to email myself (also a DH account) it went through fine. Receiving mail is no problem either.
I've had a look on the wiki, and also on the forums here, but I'm no further forward. Any help would be much appreciated!
edit: it appears to be fixed - I checked the "use name and password" and used the m123456 number.