In each client, there is a Contacts tab where you can manage all the contacts for this client.

You can easily add a new contact (using the 'Add a contact' button) or edit the existing ones (by clicking on the three dots at the end of each line).

That's also in this popup that you can define if these contacts should automatically receive the invoices and quotes you send to this client. For this, simply select the corresponding option.

Then, once you're in the document itself, you can easily send this document to the contacts concerned by clicking on Actions > Send email.
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You'll arrive in the sending screen, where you see that the contacts concerned are ticked (depending on the type of document and on what you defined at the client level). You can of course untick them and add a new contact (with an automatic update at the client level).
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You can manage all your contacts for a client in the Contacts tab of this client.

You can easily add a new contact (using the Add button) or edit the existing ones (using the pencil button). That's also in this tab that you can define if these contacts should automatically receive the invoices and quotes you send using LSP.expert. You simply have to tick the corresponding box after clicking on the pencil. The little envelope is green when they should receive these documents, and red when they shouldn't.
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Then, once you're in the document itself, you can easily send this document to the contacts concerned by clicking on Actions > Email invoice/quote.

You'll arrive in the sending screen, where you see that the contacts concerned are ticked (depending on the type of document and on what you defined at the client level). You can of course untick them and add a new contact (with an automatic update at the client level).