Sitemorse only ever makes a single connection to a mail server, returning any error message encountered. If Sitemorse experiences an error during an email transaction there is every reason to believe a user would experience the same fault and be unable to send email.

If this error references a backup (or secondary) mail server email will probably be delivered as expected to your domain. In the event the primary sever cannot be contacted a client will contact the backup server, and receive the error message. Every mail server advertised as accepting mail for your domain must be able to do so.

The error message "Too many concurrent SMTP connections" is generated by the message transfer agent (MTA) running on the mail server.

The popular MTA "EXIM" returns this message when under heavy load from all domains it accepts mail for. Capacity on such a mail server should be increased.