Hello KaiLong,
Thanks for the inquiry. As far as I'm aware we have not received any reports with IMAP connectivity issues when using an iPhone 6x model. Although you are using a legacy version of SmarterMail we would not be in a position to troubleshoot or correct that specific version if there was an issue found. If you would provide us with a sample of the IMAP logs we may be able to see if there is any connectivity messages.
I also suggest to upgrade to the current build of SmarterMail 7x that you are allowed if possible. If you are able to obtain a license for SmarterMail 13x please let us know if you observe the same behavior.
Thank You.