Introduction

Roundcube can keep opening normally after a host change while every sent message fails in the SMTP step. That usually means IMAP login still works, but the outgoing mail path no longer matches the new server environment. The SMTP host may still point to the old provider, the new server may not be allowed to relay through that SMTP service, or the port and encryption settings may no longer fit. The safest fix is to verify the full sending path from the new Roundcube host to the intended SMTP server instead of only checking whether users can log in to webmail.

Symptoms

  • Roundcube logs in normally, but sending returns an SMTP or connection error
  • Messages stay in Drafts or fail immediately when clicking Send
  • Webmail works after the migration, but outbound mail stopped only after the host change
  • Some accounts cannot send even though mailbox passwords are correct
  • The issue started after moving hosting, changing IPs, or switching mail providers

Common Causes

  • Roundcube is still configured to use the previous SMTP hostname or relay
  • The new host cannot reach the SMTP port because of firewall or hosting-provider egress blocks
  • SMTP authentication settings no longer match the mailbox or relay policy
  • TLS, SSL, or certificate validation fails against the configured SMTP hostname
  • The mail provider only allows SMTP relay from approved IPs or its own network

Step-by-Step Fix

  1. Confirm which SMTP service Roundcube is supposed to use now, because after a host change the website host, mailbox host, and SMTP relay are often no longer the same system.
  2. Compare the active Roundcube SMTP hostname, port, encryption mode, and username format with the current requirements of the intended mail provider.
  3. Check whether Roundcube is still pointing to an old server name, old relay, or old localhost-based mail path that no longer exists on the new host.
  4. Verify that the new server can reach the SMTP destination on the required port, since many hosts restrict outbound mail on common SMTP ports after migrations.
  5. Review whether the SMTP service requires mailbox credentials, a dedicated relay account, or IP-based authorization, and make sure the new host matches that policy.
  6. Check for TLS or certificate mismatch problems, especially if the configured SMTP hostname changed, the provider certificate no longer matches, or the old hostname was copied into the new setup.
  7. Test sending with one known-good mailbox and one external recipient so you can separate account-specific failures from server-wide SMTP routing problems.
  8. Review Roundcube and mail-provider error details together, because authentication failures, relay denied responses, and connection timeouts each point to a different fix path.
  9. Document the final SMTP endpoint, auth method, allowed ports, and host-level relay requirements so the next hosting move does not break Roundcube sending again.