A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Try inserting a mailto: hyperlink in a worksheet cell.
Does clicking that link launch your mail client?
If so you can do Sheets("Whatever").Range("WhereTheHyperlinkIs").Hyperlinks(1).Follow
(change the sheet name and range name/address obviously) to launch the mail client programmatically.
If not, it would be worth checking that your Mailto: protocol is associated with the correct program. (e.g. Internet Explorer > Internet Options > Programs > Set Programs > Associate a protocol with a program > Protocols: MailTo