This has nothing to do with the plugins. Don't just throw in a bunch of them.
Mailto wants an URI schema. So, you need to URI encode it, which is important especially if you have blanks, quotes or ampersands in the query string values.
Thanks Kerri, I gave that a shot. It fixed the IOS app, but broke the Android app! (both using cli-6.3). I think I'm better off just sticking with cli-5.2.
Here's how I have the email composer plugin implemented:
added the following to config.xml
<plugin name="cordova-plugin-email-composer" source="npm" spec="~0.8.3"/>
subject: 'Request for quote',
I just realized this is a thread that had been rejected. interesting. Anyway sorry for the duplicate post that is also in the phonegap build forum, window.location.href = "mailto:..." not working in IOS, but it does in Android.