IMAP protocol doesn't support this directly, but it can be done this way:

1. Call Imap.CopyMessage to copy a message to another IMAP folder.
2. Then call Imap.DeleteMessage to mark the message in the original folder as deleted.
3. To actually remove the original message, call Imap.Purge (be careful, this will purge any other messages marked as "deleted" as well)

Sample code:

C#:

// initialize an IMAP session
Imap client = new Imap();
...

// select the source folder
client.SelectFolder("Inbox");

// copy the message to the target folder
client.CopyMessage(uniqueId, "Archive");

// mark the original message as "deleted"
client.DeleteMessage(uniqueId);

// purge deleted messages (be careful, this will purge
// any other messages marked as "deleted" as well)
client.Purge();

VB.NET:

''# initialize an IMAP session
Dim client As New Imap
...

''# select the source folder
client.SelectFolder("Inbox")

''# copy the message to the target folder
client.CopyMessage(uniqueId, "Archive")

''# mark the original message as "deleted"
client.DeleteMessage(uniqueId)

''# purge deleted messages (be careful, this will purge
''# any other messages marked as "deleted" as well)
client.Purge()