I have been reading Cicero's letters in translation on the Perseus.uchicago.edu site, but check the Latin to improve my limited ability. Most of the letters include S. P. D. in the salutation, and I am wondering — is this formulaic and what does it mean?
SPD is likely an acronym for Salutem Plurimam Dicit. When used in the phrase
[Person X] salutem plurimam dicit [Person Y]
it literally becomes
Person X sends many greetings to Person Y
Person X would be the sender/writer of the letter, and Person Y would be the recipient.
For more on Latin letter-writing, see this page.
I'll just expand slightly on @HDE226868's excellent and correct answer and say that the literal translation of salutem plurimam dicit is "says very much health."
Another version you're likely to see is SQPD, which stands for salutem quam plurimam dicit, or "says as much health as possible."
Another set of abbreviations that crops up often is SVBEEV or variants thereof. This stands for Si vales, bene est; ego valeo or "If you are well, that is good; I am well." It's sort of the Latin equivalent of "How are you? I am fine."
Letters often close with just vale, which means "be well," but sometimes you'll see cura ut valeas, which means "take care to be well" or "see that you are well."