[Dovecot] LDA/Sieve and vacation: empty envelope sender

Hello everyone, while testing a vacation/reject sieve rule I get:

lda(sender@mydomain.xyz): Error: sieve: envelope sender address '<>@mail.domain.xyz' is unparsable

follows an empty

Return-Path: <"<>"@mail.domain.xyz>

and the reject of the message itself by the recipient's MTA (syntax error).

Unfortunately I had no luck when googling for similar issues, after a couple of hours of debugging.

qmail/vpopmail dovecot 2.0.12 pigeonhole-0.2.3

Thanks in advance for any hint Roberto Puzzanghera