The same email that I sent the oath form. The first day I sent my oath form and I did it twice because I wasn't sure if they received it the first time and my second email I told them to ignore my email if they already received my form before and someone was really nice (TS inicials)and told me that they received my two emails and that a officer will work on that, and congratulations for becoming citizenship etc. For that reason I just wrote to them again I said thank you and I asked if they know if my certificate has been sent already and If not, if I have an option of picking it up or paying for express shipping. Just like a kind a message and he/she were really nice and answered me