The Email cover code that appears on the Print Document screen is simply what was used the last time documents were emailed, which may be never. 'Use selected spec' also uses the selected email cover.
If you set a particular email cover on a customer, you have to choose the 'Use customer specification' to get the customer email cover as well. This is your solution.
If you choose 'use document spec', this also uses the email cover saved on the document, which may be different than the customer and/or the selected cover.
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Regards,
Softrak Tech Support