Subdomains are fine. Newsletter.mycompany.com and application.mycompany.com or even mycompany.com.

Most of time, I would use mycompany.com for company stuff and mycompany.net for Application DNS and EMails.

You've got it - subdomains are sufficient (IIRC). Something like notifications.mycompany.com for the important stuff and news.mycompany.com for the more marketing stuff (and of course someone.else.entirely for the the cold email list "well, surely this one little list couldn't hurt" ... nah, who am I kidding, never use those)