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For a new message (un-seen), ImapMessageInfo.Flags is shown an empty.
For a message that is read (seen), ImapMessageInfo.Flags will show “Readed”.
If you want to retrieve un-seen messages, please check ImapMessageInfo.Readed property for false value.
if (msgInfo.Readed == false)
// do something with un-seen messages
If a message is un-seen, it means its flags are empty. And if you want to mark it as seen, then you need to add “Readed” flag to it by calling ImapClient.AddMessageFlags(msgInfo.SequenceNumber, ImapMessageFlags.Readed);