It is not illegal in all circumstances. It is the responsibility of the sender to ensure they are using the correct information and correct process for the purpose they are sending emails. In 'Write' window using 'Contacts Sidebar' * select address book * select mailing list name * click on 'Add to Bcc' button Another method you maybe interested in is using an Addon called 'Mail Merge'. This uses a Template email which you can modify content. You can use a .csv file or create an address book specifically with required contacts. This facilitates a more personalised option and although you only create one email, Mail Merge creates one individual email to each contact using the TO field and allows you use 'Send Later' to temp store in the 'Outbox' folder for sending later. Read more info on this via the link below. Mail Merge addon extension: * https://addons.thunderbird.net/en-US/thunderbird/addon/mail-merge/

There is also a version of the [https://addons.thunderbird.net/en-US/thunderbird/addon/use-bcc-instead/ Use Bcc Instead] add-on [https://github.com/revad/use_bcc_instead_A for TB 60].