I would send a fax to mississauga that you have received an email from CPP-O and you have already emailed them the copy of receipt. Sending a letter by mail would take ages to be opened by CPC-MShuga said:i think e-mail should be enough...
http://www.cic.gc.ca/english/immigrate/sponsor/relatives-apply-after.asp
email to: CPCM-EXTCOM @ cic.gc.ca
subject: Payment of RPRF