Problem Description
- You're using Roundcube webmail running on the same server as your SMTP server (localhost connection). Note: Roundcube is not our default webmail for shared/regular hosting, but it is if you have a VPS with Plesk.
- You can login to webmail successfully, but sending an email results in a long (60s) delay and then the error: cannot connect to server
- You use CloudFlare
- The Roundcube logs will indicate the following error: Invalid Compose ID
Problem Resolution
This bug has been described many times over the years and is tracked in this Roundcube GitHub Issue.
As of May 2026, the bug appears to be resolved for all scenarios except when using CloudFlare and the IP address is proxied.
To resolve the problem you will need to remove the Proxy for the subdomain webmail.<your_domain>. After doing so it might take a few minutes, or even a few hours, before your local system sees the changes. We recommend waiting at least 10 minutes, restarting your browser, then logging in to webmail again before testing. If the issue continues, wait a few hours before trying again.
