[fetchmail]Using LMTP

I'm trying to deliver mail directly to the Cyrus IMAP LMTP socket, thus 
saving some processing time.
However, then the LMTP EOM is reached, Cyrus LMTP receiver sends a 250 
OK reply, but fetchmail insist on a 503 reply (????).
To my best knowledge, the 250 OK is fine.

Do I need anything special to use the LMTP socket directly?

Thanks,

Arnaldo