When the indirect object comes first, you don't use "to". I'll send her the letter.
Regarding your two sentences in bold, which are a bit odd but could be possibe, "to" is 'optional' but I would use 'to" in the first one.
If you include a direct object, like 'letters', you need "to" in both sentences.
Years ago you started writing letters to a penfriend.
You still write letters to each other now.
- For Teachers