We're getting this error when a multi threaded app send mail using Smtp. The solution I received was to ditch SMTP and use sendmail instead. But that's not an advisable solution in our case. Another solution is to use mailing list, but as every email is customised that's also not an option and also from email to email the subset of users may vary so creating groups based on the users are also not an option. Can anyone help with this?
My only option is to allow as many concurrent SMTP connections as possible in the zimbra server.