Definitely, the chances of your email getting blocked could be that the DNS has started supporting DKIM and the message does not bear any DKIM signatures. *** DMARC is new***** which incorporates policies for both SPF failures and DKIM failures. Due to increasing email scam, the network security is stronger than before.
Before we could send emails from PHP or any language by just mentioning From and To address but now due to new DKIM and SPF security the emails get rejected.
Please double check CodeIgniter code, if SMTP configuration is provided. This should solve the problem. In case you need my help please initiate chat to discuss further.
Thanks.