Hi Mark,
Good morning.
1. First, please check note "1746385 - Main WEBCUIF Browser Support Note".
For the crm release of your system, in order to use IE11, you must apply following notes which are the prerequisite of using IE11.
1898295
2026791
2. As note 2026791 mentioned, you need to also apply all IE11-related Notes listed in the "References" section of this Note 2026791.
3.You need to apply following notes to support email BTF editor for IE11.
2053131 BTF editor functionality for IE 11
2101913 Application freezes on IE11. F5 or refresh button restores it.
2168635 BTF Editor - Insertion of standard texts fails on empty document ie11
2111225 Backspace causes web page expiration
2110582 BTF Editor: Issues clicking on links
2106353 Insert standard text in ie11 is unresponsive
2103999 In 1 to 1 email BTF becomes unresponsive
2095537 Cursor position missing causing page to freeze
2080061 When font dropdown is missing, causes endless loop
4.I also recommend you to read following SAP KBA
2097237 - Usage of IE11 for CRM Interaction Center
Have a nice day!
Best Regards,
Bruce