Please log in to your GetResponse account and ensure that it is set for single optin. You can find out exactly how to set that up here:
You may want to check with GetResponse Support directly. They may be able to look into this further for you.
Something else to consider is that GetResponse has become massively aggressive with their suppression lists, so they are rejecting emails at a much larger rate than we are used to. Lists are being suppressed on average by 25%.