There may be occasions when you need to communicate important information with all your clients, regardless of their marketing preferences (e.g. a change of clinic address).
Create your message by creating an email marketing campaign.
The last step of that article is:
"The campaign will only be sent to those who have opted into receiving marketing messages. In the very last pop-up, select Ignore client marketing preferences to send the message to all. Be very careful about using this option due to GDPR rules."