I ran your regex over my sendmail logs and it behaved pretty much as expected. The only lines it missed were emails directed at multiple recipients, like so:

Jul 19 02:43:29 zoom1 sendmail[26193]: CAA26174: to=<XXX@aol.com>,<YYY@aol.com>,<ZZZ@aol.com>, ctladdr=<XXX@TelePath.Com> (13408/40), delay=00:01:14, xdelay=00:00:01, mailer=esmtp, relay=zd.mx.aol.com. [152.163.224.101], stat=Sent (OK)

Modifying the regex a bit cleared that up and I didn't get any more anomalous behavior. Here's the test code I used:

#!/usr/bin/perl

while (<STDIN>)
{
        # Only match lines that have a " to=" in them.
        # The leading space is important because many
        # lines have a "proto="
        if (/ to=/)
        {
                #($to_addr = $_) =~ s/.* to=([^,]+), .*/$1/;
                ($to_addr = $_) =~ s/.* to=(.+?), .*/$1/;
                print "$to_addr";
        }
}
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-Matt