Is this a correct way to use polymorphism

Is this a correct way to use polymorphism

I have a couple of classes in my system
//this was not made an Interface for some WCF reasons
public abstract class BaseTransmission
{
protected internal abstract string Transmit();
//other common properties go here
}
And a few child classes like
public class EmailTransmission : BaseTransmission
{
//This property is added separately by each child class
public EmailMetadata Metadata { get; set; }
protected internal override string Transmit()
{
//verify email address or throw
if (!Metadata.VerifyMetadata())
{
throw new Exception();
}
}
}
Elsewhere, I have created a method with signature
Transmit(BaseTransmission transmission). I am calling this method from
another part of my code like this:
TransService svc = new TransService();
EmailTransmission emailTrans = new EmailTransmission(); // this inherits
from BaseTransmission
svc.Transmit(emailTrans);
This solves my purpose. But usually when I see examples of polymorphism, I
always see that the reference type is base class type and it points to an
instance of child class type. So usually in typical examples of
polymorphism
EmailTransmission emailTrans = new EmailTransmission();
will usually be
BaseTransmission emailTrans = new EmailTransmission();
I cannot do this because EmailTransmission EmailMetadata is different from
lets say FaxMetadata. So if I declare the reference to be of
BaseTranmission type and point it to an instance of EmailTranmission type,
I lose access to the EmailMetadata property of the EmailTransmission.
I want to know whether what I am doing above is a misuse of polymorphism
and whether it 'breaks' polymorphism in some way. And if it is abusing
polymorphism, whats the right way to do this.