It appears the free Outlook app for iOS and Android phone does not support newer SNI based certificates at this time such as Let's Encrypt. If you are receiving the error message "Unable to login, Secure connection unavailable. Would you like to login anyway?" you have verified DNS is setup properly, along with login credentials and can connect fine with other mail apps like Thunderbird, this may be the case. It then comes down to and you do not require https://
for the website with Let's Encrypt, can use a purchased certificate, or you can switch autossl to Comodo. If this is the case, and you have admin rights, you can jump into your web host manager, select Manage SSL Hosts, scroll to the domain and make sure "Let's Encrypt" is not assigned to the host. If it is, delete it. It will default back to the server's default SSL. You may also switch the SSL to Comodo/cPanel, Inc certificate and it should work properly. If you do not have these rights contact us and we will be happy to help!