I found the answer. The best solution for me was to make use of the multiple
user mailboxes (Section 41 of the Exim Specification)
Add the following entry in /etc/aliases of:
#/etc/aliases
jill: fred-jillsmails
fred-jullsmails: fred-jillsmails
And in the exim configuration file
userforward:
suffix = -jillsmails
suffix_optional
In freds .forward file:
if $local_part_suffix contains "-jillsmails"
then
save /home/fred/Maildir/.afolder/
endif
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