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Thank you both for looking into this.
Doing some further research their appears to be some kind of problem with Digital Ocean (the cloud provider I use for my docker instance) and STMP. While I could not find anything concrete, other users mentioned similar problems trying to STMP to Amazon from that provider.
I resolved my problem by using the free tier of a service called STMP2Go, which acts as a STMP relay for my Calibre instance. It works very well, and I have not had a problem since.
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