Wrong rules in .procmailrc when setup Email Filters

Many time I try to setup email filter, that filters messages to ".*@.+\.mydomain.com" to imap subfolder. Each time it applies on all mail, but it must not apply to "myname@mydomain.com" mail and other!

I setup filter via usermin as "Header To must contain \@.+\.mydomain\.com", and it creates string in .procmailrc:
* To: .*\@.+\.mydomain\.com.*

The problem is in missed "^" symbol before header field id "To:" in .procmailrc, because without it this string matches to "In-Reply-To: ".

Working string is:
* ^To: .*\@.+\.mydomain\.com.*

In all examples I see that procmail filters uses "^" symbol before header name.
And, I think, usermin must add it every time for every header field.