Yahoo Sent Mail folder: recipients' names all changed to their email addresses

I installed eM Client and synced with my Yahoo Mail. Now, when I check my mail on the website, in the Sent folder, all the recipients’ names have been changed to their email addresses, making it very hard to find specific emails. The list in the Sent folder goes back to 1995. This is terribly distressing. What can I do to rectify this situation?

Hello Francis,

have you tried to contact Yahoo support and discuss this issue with them?
We cannot make or see any changes made to your account from their end or affect Yahoo server from our end.