Apple Mail quit unexpectedly

Mail is behaving strangely, I can launch Mail with the shift key so no mailbox is selected. Upon launch, I can select Inbox to show mail headers, but when any inbox message is selected or opened, the app quits. I can open and manipulate any other messages in all other mailboxes with no problem. I have tried removing the container files com.apple.mail and com.apple.MailServiceAgent, also tried removing the preference files com.apple.mail.plist and com.apple.mail-shared.plist , with the same results. Booting in safe mode makes no difference. Additionally, the files com.apple.MailServiceAgent and both preference files have not reappeared in their respective folders. I am able to access my e-mail through my internet provider's webmail page, so I am trying to keep the server from becoming loaded with mail while I try to get some advice. I'm running El Capitan 10.11.6 on a mid-2007 iMac, so there's no updating possible. Anybody?