Apple Mail

I haven't logged in to this forum in a few years but it's changed drastically and it doesn't seem it is for the better. It's more difficult to find which forum to post in. Anyway, here's my question . . .

I haven't used my Macbook Pro (2009) in a while and I just tried to check email. I have quite a few email addresses, all Gmail, but only two of them are going online and dowloading mail. I can't fathom what the problem is because the passwords are correct. I have proved that by logging in directly to the Gmail website.

Why is the mail not downloading and the email address greyed out and showing a triangle?