When sending email using our service, it's necessary to have the outgoing (SMTP) connection authenticate itself as being permitted to send mail. Using an iPhone to send email requires an additional step when setting things up, in order to do that verification. This article assumes you've already seen and entered the settings for your email account found within your Cpanel account, under "mail accounts".
When editing your email account on the iPhone, you need to find the "Outgoing mail server" section and tap the settings for the primary mail server.
In this section, it's likely marked that the "Username" and "Password" are optional; they are in fact not, and required. You must include them here and have authentication set to "password". If you don't do this, outgoing mail will only work for a short while after you've already checked incoming mail, and you will see errors about the server "rejecting the recipient's address" or "rejecting the user" and so on.
Should you still be having any difficulty, please open a support ticket with us and we'd be happy to assist in any way we can.