You can use any modern mail client with i-platform and it will work just fine.
Use the following settings for POP3 or IMAP protocol (incoming mail):
Server Name: mail.<domain.name>
Port: 995 for POP3 or 993 for IMAP
User Name: <username>
Authentication method: Normal password
Connection Security: SSL/TLS
Use the following settings for SMTP protocol (incoming mail):
Server Name: mail.<domain.name>
Port: 587
User Name: <username>
Authentication method: Normal password
Connection Security: STARTTLS
Of course, you will also need to type in the password. Please note that you have received <username> and <domain.name> at the your account was created and that your email address is <username>@<domain.name>.
XMPP
There are few XMPP (jabber) clients for iPhone. To set them up, you will only need to type in your email address (<username>@<domain.name>) and password.
Siskin IM (https://siskin.im/) supports OMEMO end-to-end encryption and has ability to exchange messages, images, files and place audio and video calls. It's been tested and it works perfectly well with our server.
Monal (https://monal-im.org/) supports OMEMO end-to-end encryption and has ability to exchange messages, images, files and place audio calls BUT unfortunately not the video calls.
Chat Secure (https://chatsecure.org/) is perfect if you only need a chat client with high security but no audio and video calls.
