The proper way to sign off an email is to use a closing salutation that appropriately reflects the relationship between the writer and the correspondent. For an email to a friend, a sign off can be casual, such as "Cheers." When closing an email to a boss or business partner, "Kind Regards" or "Thank You" are appropriate.
In formal business settings, "Sincerely" and "Yours Truly" are appropriate to close an email without sounding offensive and unprofessional. These are the default salutations for writers who have no prior knowledge or connection to their correspondent. Variations of "Thanks" and "Warm Regards," while also suitable for businesses, have a more casual tone and should only be used for email recipients with established correspondence.
Emails addressed to friends and families are more personal and suitable for colloquial salutations, such as "Cheers," "Love" and "XOXO." Other acceptable sign offs for informal settings are "Talk soon," "Cheers" and "Best." Each of these salutations has different implications and must still fit with the type of email being written.
Never send an email without a closing salutation, as this could leave the recipient expecting a continuation to the email. For business emails, include contact information, a name, business title and phone number. If unsure, always mind the context of the message when writing and closing an email.