apple mail accounts

Hi, we have annoying situation where some of our subscribers with icloud.com, me.com and mac.com addresses are complaining that our emails are display as code. From what I can gather, the email displays fine on a desktop and the issue appears to be when they view on an iphone. The emails are...

When I delete an email on my iPhone, it does not get deleted on my iMac. In Mail preferences, under Accounts, it shows iCloud IMAP (inactive). Maybe this is the problem. I don't know what to do to make it active. Would appreciate any help. Thank you.