Procmail not sending via Sendmail

I've been running a Procmail script for 18 months and it's just stopped
working. Not sure why - maybe my web-hoster has changed something (I've
got a request in).

The procmail.log file shows that the rule is being invoked correctly,
but the message the rule used to send is no longer sent. I've run
Sendmail from the command line, and it seems to work. Sendmail logs are
not verbose enough to tell if Sendmail is invoked from the Procmail
rule.

Rule is:

:0
* ^TO_me@mydomain\.com
| formail -k -X "MIME-Version:" -X "Content-Type:" -X
"Content-Transfer-Encoding:" | formail -I "From: Coordinator
" -I "Subject: Message from Coordinator" -I
"To: you@anotherdomain.com" | $SENDMAIL $SENDMAILFLAGS
you@anotherdomain.com

Any ideas what is wrong - or even how I could get about debugging the
problem? As the rule is being run, and the sendmail logs don't show any
e-mail traffic (apart from users connecting to mail boxes), I'm stuck.
Can't change the Sendmail LogLevel as sendmail.cf is not available to
me.