A valid email address is required for account activation. Do not use a throwaway email address as your account may become disabled. You must click on the activation link in the email before forum posts can be made, or files can be downloaded. Unverified accounts will be deleted after one week.
Does your email server forward mail to another email address like Gmail? If so, your forwarding mail server may be breaking the integrity of the email which causes it to be rejected by the final email server. Some more information here. Please ensure that forwarding is set up correctly, or that you do not use a forwarding email address.