In Chinese culture, politeness is never too much only except for between really intimate friends or lovers. Especially when getting along with an elderly person, it's a good idea to keep being formal and polite until you're completely certain that it's not necessary.
This link explains the conventions very well. To cite the essence of it,
When in doubt, be as polite as possible. When I feel obligated by the culture, I would simply use formulaic
您好 as 问候语 and
此致敬礼 as 结尾.
Adding translation for the citation with my editorial in parenthesis. The translation is done in a kinda verbatim way in order to preserve the sense of the culture, although it may sound unnatural in English.
2 . Greetings
Greetings should be placed on the second line (first line is title) with an indentation of two spaces.
Use polite language to make the recipient feel warm and respected.
For elderly recipients, send regards for health; for middle-aged person, career and family; for young person, love life (only appropriate if the sender is an elder to the recipient) and studying; for children, health and striving.
4 . Ending
To show politeness, use some blessing words based on who the recipient is (social status and relationship to the sender).
A general one is “此致”“敬礼” (usage similar to "best regards" in English, literal translation should be "hereby, salute"). The format convention is to put "此致" on a new line with an indentation of two spaces and "敬礼" on the next line without indentation.
People usually say "祝您健康长寿” (wish you health and longevity) to elderly people, "祝工作顺利" (wish you a successful career) to friends and "祝学习进步" (wish you good progress in studies) to the younger generation.