Entity framework use navigation properties on newly created entity
public MailMessage CreateEmail(Email email)
    {
        var tmpEmail = db.Set<Email>().Create();
        db.Emails.Add(tmpEmail);
        db.Entry<Email>(tmpEmail).CurrentValues.SetValues(email);
        db.SaveChanges();
        email = tmpEmail;

public MailMessage CreateEmail(Email email)
{
    db.Emails.Add(email);
    db.SaveChanges();

    db.Entry(email).Reference(e => e.FromEmailAddress).Load();
    db.Entry(email).Reference(e => e.ToEmailAddress).Load();

    //...
}