Reference Introduction to IRM for email messages:
Download permissions
The first time that you try to open an email message that uses restricted permission, you are then connected to a licensing server. If the licensing server verifies your credentials, it issues a use license, which defines the level of access that you have to a file. This process is required for each file that uses restricted permission. In other words, content with restricted permission can’t be opened if the licensing server hasn’t issued you a use license for that content.
Downloading permissions requires that Microsoft Office send your credentials (which includes your email address) and information about your permission rights to the licensing server. Information that is contained in the message isn’t sent to the licensing server. For more information, read the Privacy Statement.
The message "This service is temporarily unavailable" when connecting RMS server of H company implies that you are unable to get a "use licence". From above:
Content with restricted permission can’t be opened if the licensing server hasn’t issued you a use license for that content
View messages when IRM isn’t available
If you must read or open content with restricted permission but Microsoft Office 2010 isn’t available on the computer that you are using, you can use a web browser. If you use Outlook Web Access (OWA), you can view restricted messages on any browser.
You can use Outlook Web Access (OWA) to view the restricted messages using any browser.
Or you can view the messages in Windows Internet Explorer if you download the Rights Management Add-on for Internet Explorer. However, this doesn’t enable you to reply to, forward, copy, or print the messages. When using the Rights Management Add-on to view messages, attachments that might were sent with the message can’t be viewed.
So, you be able to at least view the message using the Rights Management Add-on for Internet Explorer. However this doesn't work for attachments.