Possible Clues:

• Professional writing?
• Messages from the heart
• Messages containing X's and O's, and what the answers to the starred clues literally describe?
• Treasured correspondences
• Appreciate the alphabet?
• Treasured correspondence
• Billets-doux ... or 18-, 24-, 37- and 50-Across all together?
• What you have to do to be a crossword fan?
• Billets-doux