unable to decrypt a mail coming from apple mail

[ng0]

Hi,

I've just got an email where the X-Mailer is Apple Mail. It adds some
bits and pieces, is a 7bit Content-Transfer-Encoding and I fail to
decrypt it with gpg --decrypt (applied to the email as a file and also
when applied to the BEGIN/END PGP block).
The key is imported, still valid and yet I get:
gpg: decryption failed: No secret key

I'm running an unaltered Guix build here:
ng0 at shadowwalker ~$ gpg --version
gpg (GnuPG) 2.1.13
libgcrypt 1.7.3


Help is welcome
-- ng0