Without breaking NextCloud? I don’t think so.
The only thing I can suggest is to educate your client base that they need to keep on top of software upgrades … 10 and 13 year old email clients? Sadly people want to cut corners whenever they can – and this is one example of the consequences. As I pointed out earlier, the depreciation was announced a year and a half ago … so your clients should be mad at their email client vendor for not warning them, not you.
Of course, they won’t see it that way.