If you're getting numerous sending failures (as opposed to bounces) when sending with an SMTP email system, here are things to check:

  1. Does your web host or server have a limit on the number of emails that can be sent? Use the 'scheduled task run limit' if so.
  2. If not, check if the 'keep connection alive' setting is checked in the email system settings. If so, then the problem is likely that the SMTP server is set to only accept a certain number of emails before the connection to it must be closed then re-opened. In this case, the host needs to remove this restriction by allowing any number of emails to be sent in a single connection. If the host cannot remove this limit then the 'keep connection alive' box must be unchecked. It should be noted that this will probably slow down the sending of email.
  3. If it is neither of two above and there are still hundreds or thousands of failures when sending with an SMTP email system, then a Sendmail email system will need to be used as we cannot troubleshoot a host's SMTP server or its configuration.