Yes, it is possible to get mails from last read, but you have to use uniqueId instead of sequence number. You can achieve that by using this piece of code:
var set = new ImapMessageSet();
set.AddRangeFrom(lastUniqueId);
client.GetMessageList(set);
Please note that sequence number is valid for current session only. If you connect again and some message is deleted in the mean time, sequence numbers are regenerated to current session again.