Problem sending mail via Mac Mail (Mac OS X, version 10.5.8)

Have you tried logging onto your G-Mail account via web browser?

1. Temporarily create a new user account on the Mac and try from that. If that works you have Mac account issues.

2. Temporarily create a new account at Gmail and try using in Mail from the original Mac account. If that works something is probably weird with the Gmail account.

I have seen iCal in Leopard completely refuse to sync with a specific Google account's calendar. A new account worked just fine. Also, the account worked fine in Snow Leopard. Your mail may be doing the same sort of thing.

What i found recently was that certain mail providers such as Hotmail and Gmail don't like the user accessing and retrieving mail via an app because this means that you aren't viewing 'The Adverts' on the page. Get the idea . If you haven't visited the site within a certain number of days they will block the account. At the moment Hotmail have blocked my account and are asking me for my Mobile number in order for them to send me an unlock code, (no chance...).

My advice, get an email address from your ISP and use that.