If you are using the booking calendar free version
Try to open the login dialog and retry the password. Then recheck what the email address is used in this email. If you will see the "Wordpress title", so then its somewhere in the settings of WordPress.
Try to open the wp_options table in your DB (using the phpMyAdmin tool or some other DB tool),
Find the option with name "admin_email" and recheck that the email for this field do not have the "WordPress" title.
If you are using the paid version of Booking Calendar, please check this:
Booking Calendar use the standard "WordPress" wp_mail function for sending of the emails, which possibly can be overridden by the other plugins or theme.
Firstly please open the Booking > Settings > Emails page and recheck that you correctly configured the emails in fields "From". that there was no any "WordPress titles". Test it.
Then please try to deactivate one by obe all other active plugins and test if its help or not, may be some other active plugin is override the wp_mail function and thats why you are having that "WordPress" title in the email template. try the same with theme.
For any versions:
Also please open your contact book at youe email account and try to delete that email contact, may be this contact already have the "WordPress" title saved in your contact book at your email account, and thats why you have that "WordPress" title in the emails.