You can send as many messages as needed, including newsletters, as long as you stay within your sending limits. You can check the limits according to your email service in the following articles:
Sending large volumes of messages (mass mailing), as well as unsolicited emails is considered spam. These, along with any other spam-like behaviors are not allowed according to our Terms of Service.
For websites built with WordPress, you can refer to this tutorial on how to send newsletters: Best WordPress Newsletter Plugins