If for some reason you prefer to email your logs to Club Log rather than uploading through the web form, simply email your log as an ADIF or LGS file attachment to firstname.lastname@example.org
The email subject and email message content are ignored: Club Log simply imports the attachment.
If you have logs for different callsigns in Clublog, QSOs uploaded by email are only applied to your "primary" callsign. In other words, you cannot directly update the log for your secondary callsigns via email. However, you could always pick a different primary callsign first, email the log, wait for it to be imported, then change your primary callsign back again.
Uploading by email is less reliable and less efficient than the web upload tools. If your aim is to reduce the data transferred, the most efficient solution is to use the web site and ZIP your files before upload.