If you currently have this setting:

'Tools' > 'Account Settings' > 'Composition & Addressing' for mail account select: 'compose messages in HTML format' click on 'OK

Then when you click on 'Write' the email will use HTML automatically. However, if you hold down 'Shift' and click on 'Write', it will auto shift to using Plain Text for that email. So, both are available without needing to swap any account settings.

This also works in reverse. So if you have not selected 'compose messages in HTML format', when you click on 'Write' , you auto get Plain Text option, but holding down 'Shift' and clicking on 'Write' will allow HTML for that email.

If you have specific people in your Address Book who always prefer to receive emails in eg: Plain Text, you can set this preference in the contacts properties. Click on 'Address book' select the address book to show contacts Double click on contact in list to open the Edit Contact info properties window. on 'Contact' tab: 'Prefer to receive messages formatted as : Select 'Plain Text' click on OK

More on Sending options. Make sure you can send in both Palin Text and HTML Tools > Options > Composition > General tab click on 'Send Options' button Select: 'Send messages in both plain text and HTML'

In same window you can set up to always send in HTML or Plain Text for specified domains. then anything created using HTML will be stripped of all formatting and sent as Plain Text for domains set to send using Plain Text. click on all the OK's to save all changes and close windows.