Hi,535 Authentication failed could occur due to many reasons. Some of them like username password incorrect, smtp user is disabled or smtp server authentication turn off etc etc. If you are sure about each instance of your email client has accurate smtp config then check with SMTP mail server. They will have logs and can help you with real root cause of it. But make sure your email service instances r verified first.

You can get service reference using request object as well.How to get OSGI Service reference in AEM?

Hi,ImageMagick app is to have pre sized images (image renditions) and load them based on need. Adobe also provide scene 7 however that needs additional license and it works pretty much ImageMagick with some additional features.What problem do you see in loading & keeping multiple renditions?

Hi,Contexthub accepts JSON Data and you can use API to load json data. Where exactly do you want to API to load store?ReferenceConfiguring ContextHub

Hi,There is a complete guide to upgrading AEM higher versions. Read couple of Adobe docs.Upgrading to AEM 6.4 This document is a very good one.https://helpx.adobe.com/experience-manager/6-4/sites/deploying/user-guide.htmlMigrate the repo - Which jar file to be used? We don't use MongoDB or S3 for Datastore.Ans: it depends on your preference and needs. MongoDB or S3 are not recommended storage for production server but it totally depends on your application need.