https://www.duocircle.com/email/mail-flow-monitoring/
Mail Flow Monitoring: Free round-trip SMTP and IMAP monitoring since 2018 | DuoCircle
Round-trip SMTP and IMAP monitoring with multi-channel alerts (email, SMS, webhook, PagerDuty, Slack). Free, continuously running since 2018. Real round-trip...
mail flow monitoringsmtp and imapround trip