Since 9 doesn’t like the altered database from 10. When I first opened 10 on my new computer, it did show all my emails and I wish I had done the export then.
Yes when you upgrade to V10 it does change the database, so you can’t open that directly from V9.
On a 3rd computer I just found I have EM Client 9.2.2157. Should I use that to export?.
So as your lifetime licence appears to be as @Gary says for Version 9, then I would first backup that version on your 3rd computer, via "Menu / Backup, and then copy the backup.zip file to an external drive.
Now “if your accounts are IMAP, or Exchange, or Office 365 or iCloud (non POP)” accounts and you don’t normally keep anything in Local folders, then just “uninstall eM Client V10 on your other computers” and and if you have Windows “don’t delete the database” when asked on the uninstall wizard.
Then rename the following eM Client V10 hidden mail database folder to eg: eM Client-Old. .
(Hidden database folder Windows)
C:\Users%username%\AppData\Roaming\eM Client
(Hidden database folder Mac)
“Users\yourusername\Library\Application Support\eM Client”
Then download the latest eM Client V9.2.2280 from the release history page and reinstall.
Next put your V9.2.2157 backup.zip on your external drive “in your same backup location folder on your other computers as your 3rd computer” which is normally by default in “Documents / eM Client”.
Lastly open eM Client and “Skip the setup” and restore your V9.2.2157 backup.zip file via “Menu / File / Restore” from your 3rd computer. Any new email messages etc since the backup will update when eM Client connects to your mail server.
Now “if you have a POP account” and you have new mail in V10 and / or any emails in Local Folders at the bottom left of eM Client, then before uninstalling V10, I would eg: Create a folder on your desktop called eg: V10 Mail . Then “drag / move any new POP mail and any POP custom mail folders that has been received since you upgraded to V10” including anything in Local folders “out of eM Client into the V10 Mail folder on your desktop” to same identical folders.
That will create .eml message files where once completed, you then uninstall V10 “as I described further up” and then once you have reinstalled V9.2.2280 and got your email all working again from your V9.2.2157 backup, you just then “drag / move” all your old POP message content from your desktop V10 Mail folder back into eM Client V9.2.2280 into the same folders as before.
Then if you want to upgrade to V10 later, all your messages will be there again, but make another manual backup first of V9.2.2280 incase if any issues when you upgrade to V10.