Chrystie
From Microsoft community here is a suggestion
reset the settings of word 2016 to default settings on MacBook - Microsoft Community
"To reset Word 2016 for Mac to the default settings, you can follow these steps:
1. Quit all Office applications.
2. Open Finder and go to ~/Library/Group Containers/UBF8T346G9.Office/User Content/Templates, move Normal.dotm to Desktop.
3. Go to ~/Library/Preferences, locate the files com.microsoft.Word.plist and com.microsoft.Office.plist, move them to the Desktop (Word will rebuild them).
4. Restart Word."
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Chrystie,
If you have never done that before, using that ~ command in the finder
Go to the Finder (after you have quit Office) and look for Go in top menu bar
You should see these options

Select Go to Folder
Then Paste into the new window the correct path of the file you are removing
~/Library/Group Containers/UBF8T346G9.Office/User Content/Templates
See this pic

Press Go
It opens up a folder called Templates
Drag out the one called Normal.dotm to the desk top
Repeat for this file
~/Library/Preferences/com.microsoft.Word.plist

Hit Go and it should take you to that file within the Preferences folder.
Drag the file to the desktop. Don't trash it yet.
Repeat the same using the following file name in the Go Folder dialog box
~/Library/Preferences/com.microsoft.Office.plist
Hit Go and it should take you to that file within the Preferences folder.
Drag that file to the desktop.
Don't trash it yet.
So you have now removed three files.
Restart your Word and see if the Send Mail is working now.
If it is, you can trash the three files on your desktop.