There appears to be an issue with the backspace key in SwiftKey when using the outlook app for emails within IOS.
To replicate the issue do the following:
Open an email sent to you.
Type some words,
Press enter TWICE (or more)
Type some more words
Now try backspace to delete those words.. It will now only delete one character per push at a time even when you hold the backspace key down,
The issue is not apparent when writing a new email, or replying to an email but you've not pressed the enter key twice.
IOS 10.3 (but was an issue before this version)
Outlook 2.20.0 (But was an issue before this version)
SwitftKey 1.6.3 (But was an issue before this version)
Any thoughts? I've raised a support query with the Outlook app team too .. but seeing as you are both MS owned, so perhaps you can talk to each other (trying not to laugh when typing that! )