Users of Office 365 and Exchange 2016 should avoid upgrading Apple mobile devices to ios 11 until a fix has been released. Currently an ios 11 device will simply say:
“Cannot Send Mail. The message was rejected by the server.”
Apple and Microsoft are working to resolve the issue and will be releasing a patch in upcoming software updates. Until then our advise is to resist the temptation to upgrade to ios 11.
If you are a customer and want to confirm if these issues affect you please don’t hesitate to contact us.
UPDATE:Apple have released ios 11.0.1 to fix this (https://support.apple.com/en-gb/HT208136)