Search function doesn't return always return all matching emails

Search function doesn't return always return all matching emails

Searching for a sender doesn't always return all the matches.  For example doing a search for an email address as sender in all folders in the format user@domain.com returned only 1 match from about 30 existing matches.  After doing a search only on user (without @domain.com) then returned all of them. Then only after that the user@domain.com returned the correct messages.