filter.d/sendmail-auth.conf: detect failures without user part

config/filter.d/sendmail-auth.conf

@@ -15,7 +15,7 @@ addr = (?:IPv6:<IP6>|<IP4>)
 prefregex = ^<F-MLFID>%(__prefix_line)s</F-MLFID><F-CONTENT>.+</F-CONTENT>$
 
 failregex = ^(\S+ )?\[%(addr)s\]( \(may be forged\))?: possible SMTP attack: command=AUTH, count=\d+$
-            ^AUTH failure \([^\)]+\):(?: [^:]+:)? (?:authentication failure|user not found): [^,]*, user=<F-USER>(?:\S+|.*?)</F-USER>, relay=(?:\S+ )?\[%(addr)s\](?: \(may be forged\))?$
+            ^AUTH failure \([^\)]+\):(?: [^:]+:)? (?:authentication failure|user not found): [^,]*, (?:user=<F-USER>(?:\S+|.*?)</F-USER>, )?relay=(?:\S+ )?\[%(addr)s\](?: \(may be forged\))?$
 ignoreregex =
 
 journalmatch = _SYSTEMD_UNIT=sendmail.service