Learn your state's laws! That is very important. 735 Ill. Comp. Stat. § 5/9-211
I understand that in IL, you can serve the five Day Notice by mailing it certified with return receipt requested. I do NOT know if your tenants will sign or not. I sometimes use envelopes that look like they are from Publishers Clearinghouse -- although those are junk mail, if they come certified, well, is that a jackpot???
I do not know if the service is valid without a receipt being signed.
IL also allows posting the Five Day Notice. i do not know the requirements for valid service by posting.
--184.6.xxx.xxx