mail sorter/archive and lock etc

Forgive me if this is a too simple question :-) --

I have been using procmail to sort out incoming mails into over 15
files, like:

:0
* ^(To|Cc)(_dot_)*sheep-l(_at_)LISTSERV(_dot_)UU(_dot_)SE
Sheep-l

:0:
* !^Precedence:.*list
* ^FROM_DAEMON|^From.*DEAD(-LETTER|MAIL)|-errors|EARNGATE|Delive?r|\
           (SMTP20|LISTPROC)|(^Subject:.*Error Condition Re:)
Mail-errors

Occasionaly, I experienced loss of a complete archive file.
I wonder if a proper lock file line for each recipe, or following
recipe would be better to prevent this?

:0
* ^(To|Cc)(_dot_)*sheep-l(_at_)LISTSERV(_dot_)UU(_dot_)SE
| cat >> Sheep-l

THANKS in advance!

Zhiliang