First of all if you email provider doesn't support secure protocols there is no way you can have a perfectly secure system, because, at some point, you will have an insecure connection.
But, in your case, there are, at least, two options to solve your problem:
- You can use a proxy
- You can redirect your email to another email provider
If you trust the connection you have between the proxy you use and Plus.net email provider this solution is good for you. The drawback is that you need to have a proxy allways up.
For this solution you can use IMAPProxy. Don't forget that if you want to send emails you also need an SMTP proxy like smtpprox. Or you can use nginx that supports both using the MailProxyModule.
The second solution is easier. You can configure your email provider (Plus.net) to forward your email to any other provider that support secure access methods, like gmail or hotmail.