mail sending from wrong Mail.app gmail account

I've got about 5 different gmail accounts in Mail.app -- two are actual gmail accounts and three are @mywebsite.net that I host through Gmail.

Up until recently everything was fine but now it seems all my mail is being sent from one gmail account, regardless of which account I choose to send from.

To the right of the "From:" dropdown menu is another dropdown, but mine is filled with indistinguishable "Gmail" names, maybe there's a way to edit those to make it more clear?

I've just spotted there's a way to set a default "From" email address in the prefs but I'd prefer not to always send from the same address... as in, that's the point of having so many emails in the first place. I've already sent out a bunch of business emails from my personal accout and confused a whole lot of people so it would be great to have this sorted soon.

Thanks,

Connor