How do I select a new SMTP "Primary Server"? On my iPhone (iOS 11.2.5), I had to change the SMTP server for my e-mail account.  I was able to add the new SMTP server, which appeared under "OTHER SMTP SERVERS".  I was also able to turn off the old SMTP server (which is shown under "PRIMARY SERVER", so I only have the new SMTP server active.  I am able to send mail successfully.

My question is: How do I switch my Primary Server to be the one that is active?  Does it even matter?
I haven't figured out a way to rearrange the servers so that a different one is the Primary, but I was able to work around it by simply tapping on the Primary server entry and change the details so that it points to the server that I want.  That way, the one I am actually using is the Primary.

As Christian said in the comments, it doesn't matter, because if I only have one server actually turned on, that is the one that Mail will use.