[exim] Taint mismatch, Ustrncpy: retry_update 826 ?

All,

I thought that I had fixed my system's issues with tainted data some
months ago but I appear to be logging one of these in paniclog for each
message processed:

2021-01-25 10:48:56 1l2yKc-0003H9-4x Taint mismatch, Ustrncpy:
retry_update 826

2021-01-25 10:58:56 1l2yKc-0003H9-4x Taint mismatch, Ustrncpy:
retry_update 826

2021-01-25 11:13:56 1l2yKc-0003H9-4x Taint mismatch, Ustrncpy:
retry_update 826

2021-01-25 11:28:57 1l2yKc-0003H9-4x Taint mismatch, Ustrncpy:
retry_update 826

Can someone give me a clue as to where to look?

Mike