SMTP Email having problem with Gmail/TLS connection

I have my site running on CodeIgniter and I am using SMTP credentials to send email. The smtp host that I am using is smtp.gmail.com and with a TLS protocol. The issue that i am having is that the emails are received sometimes and not always. Initially only one of the test gmail accounts was receiving all emails and any other account did not get email. I studied different forums and found that Gmail doesnot allow less secure apps/websites to easily send email and that the security settings on smtp account should be enabled to allow connecting less secure website. I did that and now many of the email clients (gmail/yahoo/yopmail and few domain accounts) do get email but thee are still FEW of them that does not get any single email. Any ideas why? Quick help will be highly appreciated.