Hi, besides the solution for Samsung only, both for android and iphone, you can use Nine mail. The HTML signature always works well. Tip: build a signature on your PC and send it by email to the phone,
then paste it into Nine Mail.
Indeed Nine is doing the job very well! An HTML signature, you can acknowledgment and read receipt.
The only problem I find is the inability to use Microsoft Azure Information Protection. I use MAIP to secure some sensitive matches + a tracking service. Currently, no software on Android allows me to use MAIP.
I have to use Outlook on my computer to achieve this. (As if encryption and privacy was needed at the office ....)
Brief Nine is a solution well suited to businesses using email on smartphone .... soon 2020 and MS is still unable to offer a software solution on android / ios to use "fully" our subscription O365 when not in the office....