How search iPhone sent Mail by recipient, not sender

iPhone sent mail search won’t search by recipient.

If I try to search iPhone sent mail for a recipient name containing “textstring”, Mail search only offers sender, subject or attachment name containing textstring.

Searching the sender field is pointless , as I know I am the sender.

The search I want to do in the sent mail folder is To: contains text string, but I can’t find a way to do this. 

Any suggestions would be much appreciated.