Why are some bounced emails not recognized?
MailMerge365 relies on the bounce detection of Outlook itself and will only report bounces if they are true standards conforming and permanent NDRs (Non-delivery reports).
Permanency is key as you would not want someone to be unsubscribed because their email bounced only to realize it was a temporary issue that was resolved a day later. Outlook does it's best to make sure the NDR is due to a permanent situation before reporting is as such (e.g. the receiving mail server reports the user's mailbox does not exist).
Once detected these emails are then automatically moved to a folder called "MailMerge365 Bounces" which are then picked up every couple of minutes by our systems and added to your report. Once added they are deleted from the folder. (You can always find them in your Deleted Items until you permanently delete them from there.)
Why are some bounces not being recognized?
This reliance on Outlook and permanent NDRs can lead to you seeing something that looks like bounced email but MailMerge365 cannot detect it. They will stay in your Inbox. If you determine the email to be a real bounce you can manually move the item to the "MailMerge365 Bounces" folder and the system will do it's best to recognize the bounce. But if they email does not contain the identifying markers for a MailMerge365 campaign (specifically the email headers) because the receiving server is not adhering to internet standards properly then we cannot register the bounce to the campaign. In this case the email will stay untouched.