I have a web app that handles a fair amount of traffic and sends a good deal of transactional emails, likely around 1000 a day. About 99% of the time there are no issues but about three months ago we started having an issue where emails are failing with this error:

{%Bamboo.SMTPAdapter.SMTPError{
   message: "There was a problem sending the email through SMTP.\n\nThe error is :no_more_hosts\n\nMore detail below:\n\n{:permanent_failure, 'port', :auth_failed}\n",
   raw: {:no_more_hosts, {:permanent_failure, 'port', :auth_failed}}
 },

We get this error between 0 - 20 times a day but, in almost all cases, manually sending the email again solves the problem. Again, this application has been in production, and processing about the same number of orders, since 2019 and we only started seeing this issue in April or May of this year. Any ideas?