In order for the MailStore Gateway to receive journal email over SMTP, it needs to be configured with a valid mailbox address. This would usually be in the form
mbox123456678990@yourdomain.com.
However, this must use a unique publicly accessible domain name so the mail server that is journaling to it can resolve it in DNS and so would require either a separate domain registering or a subdomain setup on an existing domain you own.
The other option is to configure MailStore Gateway to use the public IP address of the network it is installed on.
For example, if your Internet connection has a static public IP address of 123.123.123.123, then when prompted by MailStore Gateway to enter the domain use 123.123.123.123 instead. Any mailboxes will then be created in the form
mbox123456678990@123.123.123.123.
This email address can then be defined within Microsoft 365 as the journal email address.
Note that wherever possible the use of a domain name is preferred, though. The reason for this is when using a domain name, any changes of IP address that require DNS updates, automatically get reflected in Microsoft 365. However, where the use of a domain name isn’t possible for technical or operational reasons, a public IP address can still be used.