Is it Apple Mail or MiaB that can't trash a lot of messages at once?

I have a customer that has a vanity domain for her late husband to honor his gift to English grammar. She never ever checked the email address. She has passed away and I am going through the 7,000+ emails to pare it down to whatever is worth keeping.
If I try to delete too many emails at once, they delete from Apple Mail, I see the activity bar process them, and then I see then move right back into the folder. I have been having to delete or mark as junk only maybe 30 at a time to get it to work.

Is this something up with Apple Mail or something with MiaB?
If it is MiaB, is it a parameter I can modify to have it handle more of them at once?

Do you have this setup as “Exchange/ActiveSync” or did you use IMAP to set this up.

My Guess is “ActiveSync”

My experience is that the ActiveSync mechanism (which uses z-push) is flawed. I would recommend that you setup the mail account using IMAP or use the roundcube webui.

Since you’ve said Apple Mail its hard to understand if your doing this on a Desktop computer or iPhone/iPad. Either way, you can manually setup these apps to use IMAP. Details on this can be found in your admin portal https://YOURBOXDOMAIN.TLD/admin#mail-guide.

Or Admin Panel → Mail and Users → Instructions.

Fill in all fields that claim to be “optional” in the app.

I used “other mail” as the option when I created the IMAP account on my desktop.