So Mail has been running great for me for months... last night though I noticed my router's firewall was off and set it to medium (here) This morning, I was able to read emails but not send them. Is this because of the firewall or something else? I use aol and port 587 btw.
Yes, I strongly suspect that the router is not letting outbound connections on port 587 through. You should be able to change this in your advanced settings for your router's firewall.