Mailspring using iCloud IMAP fails to deliver email sent

Running Apple OS 11.4. Fresh Install of Mailspring today (6/3).

As installed, using the set-up option of iCloud (IMAP) for user name and Apple-supplied password, the email client will receive email fine, but fails to deliver email to my other email accounts, vis-a-vis gmail and protonmail.

This is consistent, over the last few days, and occurs on my Linux Laptop running POP!_OS 20.04/Ubuntu 20.10 using the same configurations.

Any ideas?