The "An existing connection was forcibly closed by the remote host" errors in the stack trace usually that when Forms attempts to send the email, the email server (or something in between Forms and the email server) is immediately closing the connection.  If this was happening 100% of the time I start looking at firewalls but you indicate it is intermittent or self resolves when you retry.  It may still be a firewall that is analyzing connections and after many connections from Forms it eventually decides that this traffic appears as an attack and starts temporarily block connections to protect itself.  Eventually it removes the block and emails start working again and then this cycle repeats.  I would check with your IT who manages your email servers mentioned in the logs and see if there is a way to whitelist your Forms server for email sending.

Another possibility is that your email server has some sort of threshold or limit as to how many emails are allowed to be sent in a specific timeframe.  For example one of Office 365's limits is 30 emails organization-wide per minute.  Again I would check with your IT who manages your email servers to see if there is any logging on the server that indicates this is the issue.

That is where I would start.