Geez, I run 7.84. This is correct - it is an option. Here is the text from the help function:
eReceipts Tab
You can use the eReceipts tab to configure and send gift receipts by email. Sending electronic receipts can be very beneficial in reducing the amount of paper and time used to return printed receipts to your donors. An eReceipts tab appears on the Receipts mail task when you select Create custom data file in the
Receipt type field on the General tab.
Using the eReceipts tab, you can create a Microsoft
Word merge template to be used as a template for a PDF (Portable Document Format) file. The PDF file is sent as an email to the client. For example, you want to send electronic receipts for all gifts you received this week. Configure eReceipts the same way you configure regular receipts for mailing a hard copy receipt. First, use the various tabs for the Receipts mail task to define your gift type, the constituents to include, and additional filters. Then, use the eReceipts tab in the Receipts mail task to configure your email settings, text, and rules for the email. Merge the receipt data into a Microsoft
Word template using the
Merge button on the eReceipts tab. During the merge process, select whether you want to mark a gift as receipted, receipted and acknowledged, or not updated once you run the receipts. Once the merge is complete, a PDF file is automatically created from the
Word document. The email is then sent automatically through SMTP, a subcomponent of Microsoft
Internet Information Services (IIS), with the PDF file inserted into the email as an attachment.
We have BBNC and use it to send out email blasts. You should probably talk with your IT staff. I'm not sure the load it would put on your servers and there are other issues to consider. (I'm IT so I'm always looking out for the IT staff.

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At this point, we still acknowlege every gift with a mailed acknowledgement/receipt - even our online gifts. I'll be interested to see if this option becomes widely accepted.