[Exim] mail list alias for virtual domains

I have several virtual domains that I receive mail for and have just tried
to pipe mail to an executable, like is done for mail list managers (mylist:
|/path/to/binary) that produces the following error:

D=virtual_alias defer (-31): pipe_transport unset in virtual_alias driver

my tests so far have been with mail going to my qualify_domain, which is
also in my local_domains list.

my directors conf entry look like this:

virtual_alias:
driver = aliasfile
domains = lsearch;/etc/exim/domains
file = /etc/exim/${domain}.aliases
search_type = lsearch
qualify_preserve_domain


I have another EXIM installation that I use for mailing lists, without doing
virtual domains, and all is working fine.

How do I make the installation with virtual domains work?

Thanks,


-Craig