Why can't I email from an invoice?

Why can't I email from an invoice?

By default in vtiger you can only email from a lead, organization, contacts or sales opportunities. However, with the EMAILMaker plug-in, it is possible to email from almost all modules. However, an email address must be available, either on the invoice itself (there should be an email field with an email address in it), or from the linked organization or contact.

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