It's possible to challenge it if you were not to show up or otherwise comply with the summons, but noncompliance with formalities is secondary. As case law says:
"The object of process is to notify a party of pending litigation in order to secure his appearance. 'In construing sufficiency of notice, courts focus not on whether the notice is formally and technically correct, but whether the object and intent of the law were substantially attained thereby.'" Professional Therapy Services, Inc. v. Signature Corp., 585 N.E.2d 1291, 223 Ill.App.3d 902, 166 Ill.Dec. 269 (Ill.App. 5 Dist. 1992).
So if you can honestly say that you never received notice in the first place, the lack of there being certified mail and regular mail could render the notice ineffective and overturn any judgment / sanction against you. If, on the other hand, you had actual notice, the case law above says that the actual notice would suffice even if the legal formalities were not followed.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (3 or more stars). Look for the stars on your screen (★★★★★). Thank you, ***** ***** luck to you!