Prior to the 1.3 upgrade, emails generated by the Subscribe2 plugin (http://wordpress.org/plugins/subscribe2/) were being sent through the SendGrid system via the plugin (v1.2). However all things remaining the same (I've done a good bit of testing), when updating sendgrid from 1.2 to 1.3, emails stop sending from Subscribe2.
Subscribe2 uses the wp_mail function, so this should work.
WordPress Core generated emails (like new user emails) will send through sendgrid using 1.3.
Not sure if this is an issue with Subscribe2 or Sendgrid, but thought I'd report it.
I've tried this on both 3.7 and 3.8, this behavior exists on both WP Versions.
My current solution is to not update to 1.3.