Thank you, eventualo!
It also worked for me coding my custom fields as placeholders in alo-easymail_custom-hooks.php:
in custom_easymail_placeholders
I define my [CUSTOM-FIELD-AS-PLACEHOLDER]
and I get the custom fields values in custom_easymail_placeholders_get_latest
in which I replace the placeholders with the values.
However calling directly the get_post_meta($newsletter->ID, 'meta_name',
in the newsletter template file I think is the smartest solution.
Thank a lot!