Adding a relevant “Re:” line here with the subject of your letter will help your recipient to know exactly what to expect as they read the letter.
I'm writing to you today in order to issue (a letter of recommendation, a thank you note, an application for XXX position, to terminate a contract, etc.) for [Candidate.FirstName][Candidate.LastName] the position you're applying for, the thing you're thanking someone for, etc.) because [Candidate.FirstName][Candidate.LastName]would be an excellent candidate, (we need to terminate our relationship, etc.).