Hi AWRneedshelp2016,
Thanks for posting on the TechNet forum.
I will give you some suggestions.
You could access your web email and delete the related emails which you don't need anymore.
Then take a look at the win10 mail client. And vice versa.
>>Recently upgraded to Windows 10 and inherited a list of tasks and problems I don't know how to solve
For windows 10 related questions, to get better help, you could ask on the forum below:
https://social.technet.microsoft.com/Forums/en-US/home?category=Windows10ITPro
>>Main issues relate to incoming email, junk email, items
This is WSUS forum, for outlook related issues, to get better help, you could ask on the forum below:
https://social.technet.microsoft.com/Forums/en-us/home?forum=outlook
Thanks again for your understanding.
If my answers provides help, please mark it as answer.
Thanks in advance.
Best regards,
Andy_Pan