Found another great example of the reason I started this blog. I was experiencing IMAP connection caching errors in Thunderbird. So I googled “plesk imap connections” and boo-ya there was the fix as the first result. Thank you Google, the internet, and kind Samaritans!
Unable to connect to your IMAP server. You may have exceeded the maximum number of connections to this server. If so, use the Advanced IMAP Server Settings dialog to reduce the number of cached connections.