Fallback and retries
What happens after a send fails — the immediate fallback attempt, two scheduled retries, and when your alert channels fire.
A message that fails to send gets three more chances, in a fixed order, before BooleanSMTP gives up on it.
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Fallback Connection. If you’ve set one (Settings → Delivery & Reliability), BooleanSMTP retries the message immediately, in the same request, through that connection.
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Retry on other connections. If that attempt also fails — or you have no Fallback Connection — and this setting is on (it is by default), the message is queued and retried on your other active connections: once after about a minute, then once more about five minutes later, up to 2 retries.
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Alerts. If nothing is left to try, the send fails for good and your alert channels fire, if you’ve set any up.
Open the message in Email Logs and look at its attempts to see each retry and what happened.