A common setup is to use crontab to schedule drush cron via drush.
In this case, failure to send mail during cron causes error messages, which causes drush to return an error code, which can cause cron to send an email to the sysadmin.
This level of alerting is unwarranted, especially as for a custom mail transport such as SwiftMailer sending via SMTP, failure to send mail can occur just for sending to an unknown mailbox.
See core issue #2349725: Core Mail class prints non-overridable errors on the page
Also swiftmailer issue #3057200: Duplicate error message
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adamps commentedPatch relies on D8.8 to work, but the code is harmless in D8.7.
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