Identifying the php scripts that are sending mail is the first step to protecting yourself against would-be spammers. Follow these steps to find the domains on which these scripts are running using Postfix:
Combine the httpdocs and httpsdocs Folders
1. Log into your Plesk Panel.
2. Click “Websites & Domains.”
3. Click “Show Advanced Operations.”
4. Click “Website Scripting and Security”
5. Uncheck the box that says something along the lines of “Compatibility mode for legacy option to separate SSL content.”
6. Click “OK.”
If you have have across this error: unable to complete request error then this solution could be what you just need to do to solve it.
1) Flush your Magento cache storage
First point of call, should be flushing your magento cache storage.