This error occurs when sending email to destination users. It is usually caused if any of the users in the destination queue's email address is missing in AD, or it can also happen if the SMTP server is offline or not responding to requests to send email.

If you repair the error, does it dissapear? In this case, it is most likely because the SMTP server was unavailable when K2 tried to send the email. There is a KB article on troubleshooting email issues, and it recommends to put the generation of email into a try..catch statement to try to send the mail a few times before throwing the error. The kb article is here:
http://kb.k2workflow.com/Articles/KB000039.aspx

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Thank you so much for the link and solution NeilM....it solves the problem.

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