The problem is with your ip address. Since you are using a shared host, the ip address may be listed as spam in the directory of yahoo and google. You can solve this problem by buying a dedicated ip from your host or by asking your hosting company to set up a google app for your website. In that case you can accesss your email id eg. email@example.com from gmail. Also the activation mail which is sent to your website members will be sent through google ip address. So your users will get the activation email on time since google ip is not listed as spam. I hope this solves your problem.