Nice. It worked but I think there is a little output error (probably not a big issue):

When I type the following command I get this output:

user'@'host:~$ doveadm -o plugin/mail_crypt_private_private_password=1234 mailbox cryptokey export -u username -U /var/vmail/domain/username/Maildir/dovecot-attributes

Folder:

Public ID: <Public key ID>

Error:

-----BEGIN PRIVATE KEY-----

<private_key>

-----END PRIVATE KEY-----

And when I type the following command I get this output:

user'@'host:~$ doveadm mailbox cryptokey export -u username -U /var/vmail/domain/username/Maildir/dovecot-attributes

Folder:

Public ID: <Public key ID>

Error: ERROR: error:03070068:bignum routines:BN_mpi2bn:encoding error

Key:

So in the first command the private key shows but it's under Error, and I believe it should be under Key. 

Thanks again.