Send Automatic Emails for Drop Shipment causes Error: Please specify an email address to send to

By design, the "Automatically Email Drop Ship P.O.s" option found under Setup > Accounting > Accounting Preferences will take effect on all vendors as long as the vendor has an e-mail address specified in their vendor record. This means that, if this option is enabled, e-mail will still be sent to the vendor even if "Send Transaction via Email" option in the vendor record is disabled as long as the e-mail address is specified in the vendor record. Moreover, if this is enabled, the system will require you to enter a vendor email address else, the error (Please specify an email address to send to) is shown.

As for the "Send Transaction via Email" option in the Vendor Record details, if this option is enabled, it will send an email to the vendor even if the "Automatically Email Drop Ship P.O.s" in the Accounting Preferences is disabled. The purpose of the "Send Transaction via Email" option in the record detail is to enable specific vendors to send transaction via email.

If you do not wish to send any particular vendor an email, you should disable "Automatically Email Drop Ship P.O.s" and enable the "Send Transaction via Email" for all other vendors whom you want to send email to.