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?