The account needs to be set up as IMAP on both, the phone, and the laptop computer.

Thanks. I've checked and it is set up as IMAP on both. Puzzled.

re:Deleting from one does not delete on the other.

Thunderbird imap account can only show what is on the server. So first we need to know what is displaying in the webmail account.

Suggest you first find out if the phone is working correctly. Delete an email via phone: Email should go into the imap TRash folder. Is the phone imap account properly putting deleted email in the correct imap Trash folder? Logon to webmail account via a browser and see if the email has been put into the Trash folder.

If deleted email is still visible in webmail account in the original folder then it will still be visible in the Thunderbird imap account in that same folder.

Report on findings.