The following options are useless for a common usage of Dolibarr. But they are usefull if you plan to host / provide Dolibarr instances as Software As A Service (SAAS) to your customers.
*To avoid bad use of the emailing module, it is
recommanded to set the option MAILING_NO_USING_PHPMAIL to 1 (This option is described previously). Also you can complete the message visible by customers when they try to setup the SMTP credentials of their Dolibarr by using the constant MAILING_SMTP_SETUP_EMAILS_FOR_QUESTIONS (This option is described previously). For example, you can point to a FAQ on your website.
*MAIN_EXTERNAL_SMTP_CLIENT_IP_ADDRESS: Set here the
ip of list of ip where your Dolibarr instances are hosted. So when your customer go into the setup area of email, a message will explain him how to setup its SPF record for its email domain. This is to avoid to have its emails, sent from its Dolibarr instance, fails due to strict antispam rule of some emailing servers .
*MAIN_ALWAYS_CREATE_LOCK_AFTER_LAST_UPGRADE: If set, when you launch upgrades of your customer instances, you are sure that the upgrade process recreate automatically the file install.lock so the administration tools like install.php and repair.php can't be used on your customer instance (until the lock file is deleted).