Selected Answer
Hi Chris and welcome to the Forum
I don't have this issue but read that 2 approaches work.
1) instead of using .Send in VBA (to send the email you created), delay slightly and use the old Sendkeys function to emulate clicking the Send button in a displayed Outlook message. Replace your line .Send with:
.Display
Application.Wait (Now + TimeValue("0:00:05"))
Application.SendKeys "%s"
(where 05 means 5 seconds, though you may be able to reduce that)
2) Use a third party tool, like the free Advanced Security for Outlook to bypass the security check (but note that particular tool has been replaced by another, Outlook Security Manager).
Suggest you try option 1 then, should that fail, option 2 (unless time is an issue e.g. when sending many emails).