In 2015 lots and lots of email apps for smartphones were available, each of them with their own unique proposition. Just to name a few of these (in unprioritized order): OWA, Inbox, CloudMagic, Gmail, Mailbox, Molto, Seed Mail, and many others. No wonder it is difficult to maneuver around and figure out which email app is the right fit for a company!
Most of them had no email signature capabilities except the possibility to add a boring text-based oneliner. Apple started the “Sent from my iPhone” trend and all of them had their own little tagline “Sent from my this and that” which in the business world is quite an unserious message to send.
The iPhone email client was the most widely used email app in 2015. The Gmail app for both iPhone and Android showed emails and especially the signature very badly. It was almost impossible to make any email signature design show up correctly on this app. Later updates solved these problems so in writing those are very good apps today.
Like Blackberry in the old days (which had literally disappeared from the market) MobileIron came out with a similar solution for businesses called Email+. This was a very interesting new addition to the email apps and an interesting new take on email apps. The email signature capabilities were not so well thought-out like many other apps, but they improved over time as well.
Bonus info: Smartwatches, augmented reality and AI started to take off in 2015.