I did write about this under another topic, moving it into its own thread so it can benefit others facing similar issue too.

Not very sure, but the isse sounds very similar to the one reported at

https://our.umbraco.org/projects/backoffice-extensions/newsletter-studio/comments/63591-SendNewsletterService-Error

.

I have tried to simulate the sending out with NS with smtp configured to fake-smtp PaperCut to total 26886 entries.

When the progress bar reached near the end, 20425

it displayed the following error message:

NewsletterStudio, SendNewsletterService: Invalid attempt to call Read when reader is closed. : at System.Data.SqlClient.SqlDataReader.TryReadInternal(Boolean setTimeout, Boolean& more)
 at System.Data.SqlClient.SqlDataReader.Read()
 at System.Data.SqlClient.SqlCommand.CompleteExecuteScalar(SqlDataReader ds, Boolean returnSqlValue)
 at System.Data.SqlClient.SqlCommand.ExecuteScalar()
 at NewsletterStudio.Infrastucture.Data.PetaPoco.Database.ExecuteScalar[T](String sql, Object[] args)
 at NewsletterStudio.Infrastucture.Data.PPDatabaseAdaptor.ExecuteScalar[T](Sql sql)
 at NewsletterStudio.Infrastucture.Data.EmailTrackingItemRepository.CountBy(Int32 status, Int32 newsletterId)
 at NewsletterStudio.Services.SendNewsletterService.SendEmails()

When I click Continue sending, it proceed with the remaining and complete at 26886 at 100%

It then shows the analytics page with "26886 Receivers"

However when I open the PaperCut folder (which was emptied prior to launching blast), I see file count of 26896 (all .emls created by papercut). Understand 1 of them is the test NS email, but the numbers doesnt add up.

Could it be in between the error message, that it "re-try" to send a message? Is there such logic during sending, which might explain the difference in numbers ?

Would appreciate if you can shed some light on this.