Retrieving emails and addresses from crashed Apple Mail

Mail (on El Capitan and a 2009 iMac) repeatedly crashes on opening. I’ve tried various bits of advice from other sites (including Apple) to no effect. The ultimate fix seems to be to reload El Capitan (not upgradeable), but I don’t want that hassle if it is not going to work.
Questions: 1. Has anyone got any tested ideas? 2. It seems as if both the old emails and my address book are embedded in the app rather than in external files - is it possible to retrieve them? (my Contacts app only contains a few of the addresses). I noticed that there were 30K saved emails shortly before the first crash - is that an issue? BTW everything else works perfectly and the iMac has never missed a beat in 11 years.

See if you can locate an old copy of Mailsteward (current version is for Sierra up) and use the free trial to see if it can access your emails. I have been using Mailsteward to archive my emails for more than 10 years - eg from Snow Leopard to Mojave:

Thanks, Michael. I didn’t know about Mailsteward. I’ve had a quick look at it online and at the rather minimalist “manual” but haven’t spotted a way to get an older version (maybe I could raise one through this site). But before I try, it seems that you have to have Mail and its mailboxes open before you can access them from Mailsteward, and my problem is that Mail opens and then promptly closes. In other words, you can’t use Mailsteward as a can-opener on the Mail app Please let me know if you know otherwise.

Mail doesn’t have to be open. In fact it is better if it is not open when Mailsteward is archiving.
I might have an old dmg file on a backup disk, if you don’t have any success with the developer.
Note that Mailsteward does not allow you to restore archived emails back into Mail. It stores the emails in a different format (mySQL) - this is actually an advantage because I can view the content of emails from previous versions of macOS/OSX.
BTW - did you try trashing Mail preferences as these can sometimes cause crashes if they get corrupted.

Hello again Michael

Thanks for that advice and for your continued patience.

Starting from the bottom - yes, I’ve tried trashing Mail preferences but continue being told that they can’t be trashed as they are required by the system. But a search for the correct one to trash brings up a long list of Exceptions Databases, none of which make much sense.

Your offer to send an old copy of Mailsteward is gratefully received. Rather than doing this over Tidbits, I suggest sending it direct to (from pmainesyd it looks like you may be just down the road - I’m in Newcastle, NSW).

This is not urgent - I have reverted to using accessing my emails using the iinet web service. But there were some old unanswered ones stored in Mail I’d like to retrieve, as well as addresses