On Monday, every single one of my subscribers who used an iCloud/Mac/Me email address was bounced (and silently unsubscribed from my Sendy list).
Apple's email support team responded:
Your email was rejected due to a temporary issue that has now been resolved. Please resend your email and it should get through. We regret the inconvenience caused.
You probably suffered the same fate as I did, and you may find your subscriber list is down.
If it's useful (and you know how to do it) then you can fix those subscribers across your Sendy installation here:
UPDATEsubscribersSET bounced=0 WHERE (email LIKE '%icloud.com' OR email LIKE '%me.com' OR email LIKE '%mac.com')