To be able to search the Royal Holloway directory from an IMAP client you have to configure it with an address book. Note: this only works for On-Premise (staff) email, not Outlook Live email.
Details of how to attach to this address book when using the Outlook client can be seen below but please note that this is not accessible from outside the firewall (off site) unless you are using CampusAnywhere (VPN).
Go to the Account Settings 'Address book' tab and select 'New'
Enter 'directory.rhul.ac.uk' for the server name, and your username and password to authenticate.
Change the connection port to 3268.
You can change the number of entries to list after a search from 100 to 200 as below.
Once complete press 'Next' and 'Finish'.