Ok try next closing eM Client and then going to “Finder / Applications” and drag the eM Client app to the bin. Then redownload Version 10.4.4867 from the release history page and reinstall and see if that fixes it.
If that still has the same issue, “suggest to try upgrading to the latest OS 26.4 Tahoe update” just officially out and see if that makes any difference incase its something to do with the OS.
If that still doesn’t make any difference and you have an IMAP, Exchange, Office 365 or iCloud account and nothing in eM Client Local Folders, then “close eM Client” and “go to the following hidden database folder” below and rename it to eg: eM Client-Current . Then reopen eM Client and setup as new to see if a new clean database makes any difference.