Test email unsuccessful, tried many things

Brand new WordPress site hosted on SiteGround, with MailPoet 3 plugin. I created a mailbox on the same domain to use as the FROM address for welcome emails and newsletters. At first, the welcome emails were being sent, but going to Spam. So I tried to improve the deliverability by editing the Send With… settings. Now I can’t even get the test email to successfully send, not even by changing the Send With… settings back to their defaults.

I read the article in MailPoet 3 Help and verified that (a) The From address matches the domain of the website (b) I have not exceeded the sending limit, since I’ve only send a handful of test emails.

Here are the error messages on the test email using various Send With… settings:

(1) Send with your host (not listed) – “Could not instantiate mail function.”

(2) Send with your host (Siteground) – “Could not instantiate mail function.”

(3) Send with SMTP – “SMTP has returned an unknown error.”
Note: I called SiteGround support and verified that all the SMTP settings are correct, including port numbers, password, security, etc. I’m able to connect my mail client to SMTP with the same settings. They checked their logs and there was no error message of a rejected SMTP connection event. They said it *should* work.

Is there a way to delete all the settings and start over without losing my email designs?