Yes, the MX record value is necessary. Using the custom sender email address feature, you will need to create a subdomain (i.e. security.tines.com) in order to get (i.e. tines.com) to work as a sender domain. The subdomain is used to verify that you own your domain, and does not actually have a mail server.
Tines' custom sender email address feature is powered by Amazon SES and MAIL FROM, and the need for the MX record is a requirement from AWS: Using a custom MAIL FROM domain.