I am a US citizen and I currently live abroad. I will work for a US company by distance and I need to complete the I-9. I have my relative's mailing address that I currently receive my mail. Which address should I put to I-9? My US address or my foreign address? Thank you in advance
You should put your own address.
Keep in mind that your employer may need to register in the jurisdiction you live in, depending on the local laws, and pay local taxes.
You should only need to complete an I-9 if you are starting work in the United States. Since you are not planning to work in the United States, the company should not ask for an I-9 from you.