Every so often we see a question from someone asking why Outlook adds an extra space following apostrophes. This isn’t an Outlook problem, but it is a sign we spend too much time in Outlook…
This is caused by Contextual spelling in Office and using French language settings (If you aren't using French language settings by default, you might be replying to messages or using templates designed by others.)
Contextual spelling picks up words that are spelled correctly but used out of context, like there and their or pair and pear. Its disabled if you have less than 1 GB of ram as it’s a resource hog. (A really convenient resource hog!)
Check in Tools, Options, Spelling, Spelling & Autocorrect Options. Look for a setting for 'Use contextual spelling' and uncheck it.
If you want to use contextual spelling, you can undo the changes it makes in error by pressing Ctrl+Z right after typing the apostrophe. Backspacing then retyping the apostrophe may also take care of it.
It's also reported the using smart quotes may affect it. If disabling contextual spelling doesn't stop the problem, disable smart quotes. This setting is in Autocorrect options, Autoformat and Autoformat as you type tabs.
I'm not an English grammar expert, let alone a French grammar expert, so the short, non-technical explanation is that it happens because in French grammar there are spaces before and/or after some punctuation marks and the spelling / grammar checker applies the French language rules.