Le sujet du mail. -- The subject of the mail (of a specific mail)
Le sujet de mail. -- The mail-subject (in general)
In some cases it is easier to see than in others.
Une chanson d'amour. -- a love song (a song about love in general).
Une chanson de l'amour. -- a song about love.
I would say that without the article, the second noun modofies the first noun. So you could say:
Une chanson d'amour est une chanson de l'amour.
The input of a native speaker would be appreciated.
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