I receive incoming emails but cannot send out. They stay in the 'Out' box. In Mail Preferences it says 'Outgoing Mail Server (SMTP) - in the slot is 'concordmedia.org.uk I am using IMAP system.
Not a lot to go on, but here are the steps I follow:
Is there a guide from your organization that explains how to configure Apple Mail or even a configuration profile so you can enter the settings perfectly without having to enter them manually?
It is very unusual for an email server's SMTP address to use the same address as its parent website. It is much more usual to use an address starting with
A DNS lookup for
smtp.concordmedia.org.uk gives different result to
concordmedia.org.uk, so I am confident that your issue is an incorrectly specified SMTP server. Try
If the problem is due to El Capitan having insufficient HTTPS support (likely outdated cipher suites), you can fix the problem by setting up a local proxy server and routing https traffic through that. https://forums.macrumors.com/threads/fixing-https-issues-on-old-versions-of-os-x.2281326/
This would be the first instance I'm aware of in which a server dropped support for all of El Capitan's cipher suites, but it's plausible, as it has happened on older versions of OS X.