Messages "sent" are not delivered to addressee

Hi. Just installed Mailspring on an iMac running OS11. Installation and synch with iCloud account seems normal and consistent with the Mailspring account I have on a Linux machine. But, when I send an email to myself-to both my iCloud account and my Protonmail account, it never arrives.

No error messages or any suggestion that there’s an IMAP/SMTP issues.