cant set gmail for my php applicaiton. Gmail prevent all signin attempts from my server
i have a php application from where users can send email notification to their customers. users can send email using their yahoo or other email hosting but they are having problem on gmail and hotmail. when they try to authenticate their gmail address gmail simply disallow it. when i check gmails device activity and notifications section for that email address, i found "Sign-in attempt prevented by Google" for all the requests from my server ip.
how can i resolve this issue?