Using Microsoft Outlook or similar email management software it should be possible to convert between RPMSG and EML email messages, simply by importing the former and saving it in the later format.
Convert Microsoft Restricted Permission Message to Email message.
We found 2 software records in the database related to .rpmsg to .eml file format conversion.
Microsoft Windows software - convert rpmsg to eml on Windows
An e-mail management tool distributed with Microsoft Office suite
Convert from rpmsg file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned rpmsg to eml. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert rpmsg to eml on OS X
Mac version of the popular e-mail client and personal information manager from Microsoft