It's not showing up as IMAP because it's a secured connection. It shows up as TLSv1, in this case. Since that traffic is encrypted, wireshark can not see what the encrypted traffic really is, so it just tells you "Some traffic, TLSv1 encrypted."
As for using telnet to do different stuff, well it depends.
Some protocols are human readable, like HTTP and SMTP. Others are partially human readable, like FTP.
Others are pretty much lost to telnet, like MySQL or any SSL encrypted connection.
As for the specific example of DHCP: I'm not sure. I think it's not a text based protocol.
If you want to be sure about any protocol, you can read the RFC (the protocol specification, if you will.)
Here's the RFC for DHCP: http://www.ietf.org/rfc/rfc2131.txt