Hi Joan
HTML will always be an issue due to how the different online mail providers read it, your best option will be to rather send it as a PDF attachment to the email
If you are using NAV 2009 and up you wont even need a PDF printer as the functionality is now built into NAV reporting
Basically the developers need to write a report in NAV that displays the content that you want the client to see, in code they need to have the report run and save as pdf, then create a mail using the NAV SMTP functionality and attach the PDF before sending
This could be run on a schedule or when a user clicks something in NAV
There may be reasons why this might not work but it is the simplest solution to eliminate formatting issues if it can be done
Happy to try answer any question is they have as i have done this many times for our clients
Cheers
Nev