I know that the term is used on the letter being written, but I'm not sure if it is required to use it on the envelope. Thank you.
2 Answers
You cannot use さん on envelopes. Just as you have to use Mr./Ms./etc on English envelopes no matter how you normally call them, you have to use 様 on Japanese envelopes. 様 is a more polite name suffix than さん. 先生 is also okay if you usually call them sensei (a teacher, lawyer, etc.). If you send a letter or a card to a kindergartner, くん/ちゃん is also acceptable.
Inside the envelope you can call them however way you think is suitable.
様 is for a person. You have to use 御中 instead of 様 when you send a mail to an organization. See: How to address someone in an email who referred to themselves as part of a department
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1I don't know about you, but I use Firstname Lastname when addressing envelopes. I've never once written "Mr./Mrs./Ms. Lastname" on an envelope.– WilliamMar 13, 2017 at 23:21
Depends on to whom you are sending the letter. In business occasions it's very common to use 様 instead of さん because it shows more respect.
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2Right -- we never use さん on envelopes, not just "common instead of" Mar 14, 2017 at 2:09