are your mail accounts online? Are they connecting? Is your mail server authentication working? That’d be where I’d look first. With those connections and authentications working I’d reboot from the SSD again. If your mail program is not updating the emails after that then there’s the chance to do some more diagnostics.
On a vaguely related Mail note, my SO asked ‘how do I attach these images to an email’. There are several choices of course. She’d tried using the photo browser after realising the file browser does not show up what’s in iPhoto. Each time we tried (photo browser, drag and drop, export and insert) Mail crashed. Unheard of! After a reboot it worked seamlessly.