Had the same issue: CRITICAL: SMTP Authentication Broken in 1.17.2 - Error 296 with Empty SASL_PATH - Bug Reports - Mailspring Community and looks like it was already reported on the day of latest update as well (just found out): Today’s update breaks SMTP AUTH after STARTTLS (IMAP works) - Bug Reports - Mailspring Community
Release 1.17.1 · Foundry376/Mailspring
Rolling back to 1.17.1 fixes the issue for now, hopefully they can fix it in the next release.
Hope that helps you out for now 