I suggest you (and your wife) do a little reading about the IMAP mail protocol, which synchronizes mail on Thunderbird and the server, and other devices if you configure them such as a phone. See https://support.mozilla.org/en-US/kb/imap-synchronization That will help in preventing nasty surprises based on assumptions. To do this in Thunderbird you would just need to add the optonline account and be aware pressing delete in Thunderbird will also affect server copies. So File menu > new > existing mail account. Many providers also support carddav for synchronizing contacts. But it appears optonline does not. Or my google search is not a good indication. In which case you will have to use whatever export optonline offers for contacts. Thunderbird can import CSV, vcard and LDIF. All of which are fairly well supported formats. I would expect optonline to offer csv as a minimum. You can then import the contact exported from optonline to Thunderbird using the import option in the address book..