Huawei P8 Lite - email client not synchronising with BT email server

Our Huawei P8 Lite (2016 Android 5) email client is not synchronising correctly with the BT mail server. It picks up all the incoming mails, but does not seem to take account of mails deleted on the server by either the iPad or under webmail. We have three 'routes' into our email - webmail from various Windoze machines, the iPad and the Huawei. The outgoing IMAP is set to port 25 while the incoming is set to port 110. 'Delete mail from server' is set to 'when I delete from inbox' Ironically, the old Prestigio (Android 4.4 KitKat) phone with exactly similar settings, except for the ports, does synchronise correctly. Most puzzled.