[exim] "taint-safe" alternative for $sender_address_domain ?

Hello,

we recently updated a pretty old exim installation.

It still had "tainted" configuration lines.

So far we had had success replacing e.g. $domain with #domain_data and $local_part with $local_part_data

However we are currently stuck on $sender_address_domain. According to the documentation there is no straight-forward replacement, e.g. $sender_address_domain_data

What is best way to go around this? The configuration line in questions looks something like this:

directory = ${lookup mysql{select wert from konstanten where name = "mailpfad" and id = 1}}/${lc:$sender_address_domain}/outgoing/


Regards,
Kristijan