is the Exchange Store Limits different when using outlook application downloaded from o365 subscription? i know its a odd question but we have this situation right now. couple of users who got the new outlook app from their O365 account, all of a sudden gets error when sending emails:
"This message could not be sent. Your server administrator has limited the number of items you can open simultaneously. Try closing messages you have opened or removing attachments and images from unsent messages you are composing. Try sending the message again later, or contact your network administrator."
all mailboxes are on Prem exchange 2016. no one else has the issue.