The error appears when the large email[s] gets stuck into the mail box associated with the email queue[s]. The help desk script tries to fetch the large email[s], however due to 'memory_limit' size limitations, it is unable to fetch the mails leading to memory exhaustion.
To resolve the issue, increase the memory_limit set in the php.ini.
The '.htaccess' or custom php.ini can also be used for this purpose e.g. you may edit the '.htaccess' as follows:
php_value memory_limit 256M
'.htaccess' directives apply only to the folder in which the '.htaccess' file resides, and all the folders inside. This means that you may edit the '.htaccess' file and only the particular site will get affected where the '.htaccess' file is residing. It will not affect the functioning of any other site on that server. So, it allows to set up a finely-grained tree of server directives, each subfolder inheriting properties from its parent.
The usage of .htaccess/custom php.ini depend upon the Server Configuration.