For security purposes, the public testrun server only allows sending encrypted emails by default. Each chatmail operator can configure whether to enable this or not for their own users.
You may send an unencrypted email directly to a given email address if you set up your own classic mailbox (take care to only use this address from Delta Chat exclusively) or you install a chatmail server and enable sending unencrpted email for yourself. It will then appear in the menu of the client. You also need to disable spam filtering to receive unencrypted email inside your Delta Chat client in such cases under 
Settings → Advanced → Relays → 
edit → 
More Options → Enforce Encryption for All Relays 
Otherwise, you can send encrypted mail to classic contacts if they already have PGP installed by creating a contact vcard for them with their email address and PGP public key included. See: