A white screen appears when there was an error 500, which is a code for "internal server error". To be able to see the actual error message you need to enable the error reporting in Joomla's global configuration. Set this setting to "Maximum".
What if you cannot login to the site?
In case Mailster is the component making trouble (e.g. because of the above described fatal out of memory error): don't worry, there is a way to login to the site by telling the Mailster plugin to stop it's execution. The Email Forwarder plugin can be disabled by putting a file "no_mst_plg_exec.txt" or "no_mst_plg_exec" into the directory [joomla]/administrator/components/com_mailster. It is not important what is in the file, but the filename has to be exactly one of the both mentioned variants.
After creating the file you should be able to login to access your site's backend, again.
Then deactivate the Mailster mail (email forwarder) plugin in the Plugin Manager (putting the status to "disabled").
If you already know that this problem is due to a very large email sitting in the inbox: directly navigate to your mailing list's settings, in the "Tools" section and use the link to delete the first email from the inbox. After fixing the error do not forget to reactivate the mail plugin.