* Formally: "Dear Mr./Ms./Mx. [Last Name]," This is always appropriate, especially in initial correspondence or formal settings.
* Semi-formally (if you've had some prior interaction): "Dear [First Name] [Last Name]," This is acceptable if you've established a slightly more relaxed professional relationship. Avoid this in your first communication.
* In conversation: "Mr./Ms./Mx. [Last Name]," or "[First Name]," depending on their preference and your established relationship.
Never: Use titles like "Esq." or "Attorney" in the salutation. While these are technically correct, they're considered somewhat outdated and overly formal in most modern contexts. Putting "Esq." after their name in a signature block is acceptable.
Ultimately, it's best to err on the side of formality, especially in your first communication. Observing how they address you in their reply will provide guidance for future interactions.