Most email clients allow their address books to be exported into LDIF format. Thunderbird can import this format easily. Check your current email client's documentation for instructions on exporting an address book.

Importing an LDIF file to Thunderbird

1. In Thunderbird, open the Tools menu and choose Import... .
2.  In the Import dialog, select Address Books and click Next.
3. Select Text file (LDIF, .tab, .csv, .txt) and click Next.
4. In the file picker dialog, choose the LDIF file you just exported.
5. You should now see the “Address successfully imported from Text file” message. You may wish to verify the import was successful by checking the Personal Address Book contents within Thunderbird.