[exim] Autoresponder limit not working correctly

Hello,

we are using an autoreply transport with exim 4.69 and according to our
configuration:

once = /vrmd/mailin/spool/once.db
once_repeat = 1d

there should be only one autoresponse per sender address and day. But we
recently had the case, that two autoresponders where sending more than
100.000 messages to each other each day. So obviously, the limit didn't
work. How does exim know which address to use as key for the once-index?
The return-path is typically empty on auto-responses.

Kind regards
Marten