Answer
This is an unusual failure mode — normally a TemporaryFailureException just retries the address according to the sender's MAX_RETRIES/RETRY_INTERVAL settings before marking the message Failed, so a thread actually getting stuck on it is worth raising as a support case if basic checks don't resolve it. Before escalating, check the Mail Sender's own TIMEOUT parameter for an unusually high value, and test the mail sender in isolation by creating a dummy routing address of type Mail, right-clicking it, and using Test to send a message directly — this isolates whether the underlying mail sender configuration itself is the problem, separate from whatever specific event/message triggered the stuck thread.