You'll have to Google that IO exception error, I haven't seen it before. I found this thread:
It's quite old, and I can't say if any of the advice would help. Whenever I've had SMTP issues with SSRS it was either a firewall blocking the port, or the SMTP service was stopped or needed restarting for some reason.
If you're still blocked on this issue after 24-48 hours, I suggest you find a command-line or PowerShell email client that can send, test that it works in your environment, then reconfigure your subscription to generate the file to a shared folder of some kind. Then use your email sender to send that as an attachment. If that works, then it will isolate the problem to the SSRS email subsystem (I'm not 100% sure but I believe it's independent of SQL DB Mail), and it's a potential alternative method of emailing in case you can't figure out the problem.
If it doesn't work, then there is a downstream issue with your SMTP. I found another link that suggested it may be a TLS configuration problem. Again, my experience has been limited to basic SMTP problems external to the SQL Server/SSRS environment.