As has already been mentioned, using the ISP's mail addresses and servers isn't recommended any more than the use of old software.
Although my PC is running Windows 7, my email is still OE6 using a Windows XP 'virtual window'. This was a feature of Win7 but is now unsupported. I have lots of active email addresses and a personal domain name. The only time I get issues is when the ISP (Virgin in my case) email gets blocked because it's been used for sending spam by someone, not me.
As you've discovered, using the webmail option often works as the ISP tend to swap the server for that first, before telling clients there is any problem.
What I've been able to do is swap the outgoing mail to my own domain name server but it can be fiddly to set up and is frowned upon by using the outgoing server for mail it's not supposed to be handling.
There is something to look at. In the OE6 settings.
Tools - Accounts - (select the email address) - Properties - (click Advanced tab)
Is the outgoing mail (SMTP) set to 25 ? Try 465
It seems that IDNet have several different email systems, each with their own settings and servers. By you swapping to the 'securemail' option you may well have invalidated your own email address. More about that at https://www.idnet.net/support/guides/email/settings.php
Virgin Cable (100/6) + EE Mobile BB