Rejected rate means the message was not accepted (blocked) due to reputation drivers such as end user complaints, spam trap hits and unknown user rates. Applying best practice recommendations listed for those drivers will help decrease the rejected rate. Content and authentication may play a role as well.
How to Resolve Sender Rejected issues
There is no single fix for problems with rejected rate. All of the following areas can cause higher rejected rates:
(Psst! We highly recommend also reading the articles about domain authentication and tracking domains!)