I just switched from Thunderbird to Em Client (Home). I am not seeing any inbox posts past one week. I have about a dozen that show in T Bird that are months old, but not in Em Client. I am saving them for various reasons.
It could be for a number of reasons. If both Thunderbird and eM Client are setup as IMAP, you should be able to see all the folders and messages in both. If Thunderbird was setup as POP3, then the older messages will not be on the server, so eM Client will not be able to download them.
They could also be in Local Folders if you imported from Thunderbird when you initially setup eM Client. If you can’t see the Local Folders, you can enable them in Menu > Tools > Settings > General > Show Local Folders.
If they are not there, you can import them again from Thunderbird. Go to Menu > File > Import > Thunderbird , and choose Import selected folders.