When do you write postcards in German, when do you start with liebe and when do you use lieber?

Let's start with the head of a German postcard.

When the recipient is a woman, use liebe,

Such as: Liebe Uta

When there are more than two recipients, use liebe regardless of gender.

Here, more than two people include the plural, such as meine liebe Tanten.

It also includes the following two names, such as: Liebe papa und mama.

When the recipient is male and singular, use Lieber.

Like my lover, dad.