It's commonly known and used in the agency and marketing world. Search for "go high level" on YouTube. Every marketer I know switched from ClickFunnels (reviewed in article) to High Level. It uses Mailgun on the backend for email delivery or can connect directly to SMTP.
If you need a CRM with AI features, calendars, newsletters, funnels, etc. then High Level is worth considering. I've been using it for a couple years and love it. For startups, it's a cheaper alternative to HubSpot.
For additional context, I've switched all my businesses and clients out of Mailchimp and Klaviyo to High Level.