Mail Problem on El Capitan

Thanks. After deleting the unread junk mail, were you able to open the Mail app without it crashing?

If you can't, press and hold a Shift key while opening the Mail app again. If it continues to crash please write back for additional suggestions.

Please open  (Apple menu) > System Preferences... > Internet Accounts. Its icon resembles a round blue @ symbol.

In the leftmost column, select the service responsible for your email account. Without revealing your email address, your name, passwords, or anything else you might consider personal, describe what you see on the right side of that window. I realize those instructions may strike you as vague but please describe as much as you can.

At your option, you may post a screenshot of that window but you must obscure the aforementioned personal information because this is a publicly available website in which personal information must never be revealed. Instructions for capturing, editing, and uploading a screenshot to this site please refer to the Appendix in Writing an effective Apple Support Communities Question - Apple Community.