XF 2.1 Swift_TransportException: Email failed: Expected response code 220 but got code "", with message "" src/vendor/swiftmailer



We've been getting this error once or twice a day for a few weeks now. We use Mailgun via SMTP to send mail, and their logs show no history of these failed messages, although they do show past and subsequent emails being successful. In fact, it's this sporadic nature of the error that's been the most confusing for us, especially since the start of these hasn't proceeded any changes or updates to our forum, add-ons, etc.

Researching the error online brings up many instances where folks had issues with encryption (we use TLS currently), but that doesn't seem like a very plausible explanation for this case since almost all our emails send fine across all email providers, and the specific email addresses that have produced these errors also have other emails that were successfully delivered.

Any suggestions or leads would be much apprechiated!
@bforesman For reference, we run Linux. Since I posted this, the issue has resolved by itself and we are no longer seeing these errors; I still don't know why we did to begin with, but I'm inclined to blame something external to our system given that nothing pertinent changed on our end across before/after the errors began/stopped appearing.
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