Users are likely to encounter an SSL Certificate Trust Warning when connecting to our mail servers using an aliased name such as `mail.youdomain.com`. This may be preventing you from connecting our mail servers to send/receive emails.
To get around this, you should connect to your mail server using its actual hostname, which will be something like `mail123.extendcp.co.uk`. Resellers can find this in their Reseller Control Centre reports section:https://customer.heartinternet.uk/manage/packages-s.cgi
Alternatively, you can add an exception to the SSL Certificate via Keychain.
See below for more infromation:https://www.heartinternet.uk/support/article/ssl-warning-on-apple-devices-when-connecting-to-mail-server.html