[exim] tainted string in 4.93

After an upgrade to 4.93 from 4.92 (FreeBSD ports), I am getting


2020-02-24 18:13:08 1j67LU-0005vG-1C == ari@??? R=localuser
T=local_delivery defer (0): Expansion of
"${local_part}${local_part_suffix}@$domain" from command
"/usr/local/libexec/dovecot/dovecot-lda -a
${local_part}${local_part_suffix}@$domain -d $local_part@$domain -f
$sender_address" in local_delivery transport failed: attempt to expand
tainted string '${local_part}${local_part_suffix}@$domain'


I don't understand how to work around this problem because there is
nothing in the manual I could find around how to mark a particular
string as not tainted, or to filter it appropriately to be safe. Any
pointers on what to do here?


Cheers

Ari