1. Create a query of constituent records linked to NetCommunity users. 
  2. Add an attribute to the records that you do not want to send email to. You may do this manually or globally. 
  3. Create a list in NetCommunity based on the query. 
  4. Create an email blast in Blackbaud NetCommunity. 
  5. Send the email. 
  6. Globally add the attribute from step 2 to all records in the query.
  7. Each time you process signups in the Plug-In, make a copy of the email blast above and resend it and use step 6 to remove the records just sent from any subsequent blasts.  
OR
 

Designate a unique constituent code for the User Login form.  When the transaction is processed in The Raiser's Edge plug-in, the 'constituent code date from' field will be the date of processing.  A query can then be created based on the 'constituent code date from,' to group the records who had been processed as of a certain date. 

*Note: this may not be 100% effective, as users in The Raiser's Edge could add the code to records from that side, so a carefully constructed procedure/policy document would be helpful to handle this.