Could you please give us the source where it is written the P.O. Box number and street number is a MUST?
Please see the following link for the Instruction guide EG7000:
http://www.cic.gc.ca/english/information/applications/guides/EG7TOC.aspUnder the Contact information for the Generic Application Form for Canada (IMM 0008), it's written:
Question 1 Indicate your current mailing address (where information should be mailed) by typing the following information:
Post Office Box (P.O. Box) number, if applicable. If you do not indicate post office box, the Street number must be provided
Apartment (Apt.) or Unit, if applicable
Street number (No.), if applicable. This must be provided if you did not type in a P.O. Box
Street name, if applicable
City or Town
From the list, select the Country of your current mailing address
Province or State
Postal code/zip code
District, if applicable
Thanks and Regards,
Ammaar