The return won't actually be rejected, but it will be subject to additional screening/processing and any refund will be delayed.
From IRS -
If you file your federal tax return without your IRS assigned IP PIN:
- Your electronic return will be rejected and you won't be able to e-File
-
Your paper return will be subject to additional screenings to validate your identity and will delay any refund you may be receiving.
https://www.irs.gov/identity-theft-fraud-scams/the-identity-protection-pin-ip-pin
