When sending to lots of addresses it is normal that not all of the email messages go through: some of the email addresses could have become inactive or be temporarily jammed, e-mail servers could be overloaded, the connection could be momentarily interrupted or slow. If the percentage of sent messages is very low, or even no message is sent, it is necessary to check the Send Settings.

Actually, it could be necessary to change the send mode (direct or through SMTP) and, if your SMTP server uses antispam filters, lower the number of connections and raise the pause between messages. The SMTP server could have restrictions that will prevent you from sending more than a certain number of messages within a specific time span (for instance a given server may not allow sending more than 50 e-mails per hour); in such case, after the first send, the operation will be completed at a later time using the History functions.

If the SMTP server antispam restrictions can’t be overridden and sending operations will go through only with very slow settings, you can try with the direct send or by using a “more tolerant” SMTP server (on the Internet it is possible to subscribe, even for a low price, to very efficient SMTP services).
If the problem arises while using the direct send mode, it may be necessary to manually specify the IP address of the DNS server.