Step 1. Go to Sentbox, see which emails have a red delivery status. Hoover over the status to check why delivery din't succeed.

Or
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Go to Exports. Download the Email History Export.  Filter the exports on bounce = 1, check reason why mailing is bounced.
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Often it's because of the spamfilter, in this case we strongly suggest to send an email again.
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Step 2. Go to Attendees, search for the attendee(s) name(s), select them and click on the bulk action > Send
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Step 3. Check status in Sentbox or exports. if it is delivered this time, no further action needs to be taken.
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If not:
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Step 4. Go to Sentbox > search for email and person you have to resend the email to.
​Copy the whole email (which includes the personal link) and sent it via your own mailing system. 

Remember: each email in the sentbox has its own magic link connected to the receiver (an email adres)  You can NOT send one email to more people. You have to copy-paste per attendee his/her personal email from the sentbox.