The 'services' tab in the Manager preferences has 2 settings that affect whether or not Helpdesk issues and/or note additions to issues or works are allowed by email:
NOTE: These settings are on the services tab because they can be set by each service area manager independently. Thus one service can allow email submissions whilst another might disallow them.
The 'messaging' tab on the Manager preferences has fields where you can specify entire domains that are allowed for email submissions in addition to a whitelist and blacklist for specific email addresses.
In each case, separate multiple domains or email addresses with commas as above.
In general, a blacklisted email address will be blocked, even if also whitelisted, so please check entries carefully!