I've been sending e-mail via my N900 for a while now, and it's always worked perfectly, but just now i've added a second e-mail account which uses POP3 (I had yahoo.uk POP3 + Gmail IMAP4 then added my school's POP3 account)

So the school account wouldn't send e-mails, but after an hour solid of tweaking, i fnally got it to work, and to test it I sent an email to my yahoo account, which received the email, but then i realised it couldn't reply, and now experiences the same issue that plagued my school email account! (SMTP authentication failed)

Is this a router issue?
a phone issue?
a modem issue?

your help's appreciated