If you are getting numerous sending failures (as opposed to bounces) when sending with an SMTP email system, the first thing to check is if your web host or server has a limit on the number of emails that can be sent out per hour and per run of the cron job for the arpReach script.
• If it does, make sure you comply with that sending limit (See How to comply with the server's sending limit per hour)
Be mindful of and set the appropriate figure on the Send limit per run setting if your server also has a specific limit on the number of emails that can be sent out per (cron) run of the arpReach script.
• If it doesn't, check if the Keep connection alive setting is checked in the email system settings
If it is, 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 your web 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 emails.
If it is neither of the two above and there are still hundreds or thousands of failures when sending with an SMTP email system, then we suggest you use the Sendmail email system for the time being while you seek assistance from your web host about the SMTP issue.
Your web host should be able to check your server logs and make adjustments to your server's SMTP configuration as necessary. Providing them of the date/time the failures occur might help.