Include all of the following.

Version, with the build stamp. Not "6.3" and not "the latest". The full four-part number as the downloads page gives it. 6.3.4 ships as build 6.3.4.43 on Windows, 6.3.7 as 6.3.7.46 and 6.3.8 as 6.3.8.47. Point releases days apart change behaviour, so the build matters.

Platform and package. Windows 10 (1607 or later), Windows 11, or Windows Server 2016 or newer. 64-bit only. On Linux, x86-64 or AArch64 with systemd, supported since 6.3.0; 6.3.4's packages are x86-64 only, and the AArch64 packages and the AppImage were last published with 6.3.3 before 6.3.6, and 6.3.6 and 6.3.7 carry them again for both architectures, the AArch64 files cross-compiled and tested under emulation. Say which package you installed: hMailServer-6.3.4-x64.exe, a .deb, an .rpm, or the AppImage; from 6.3.7 on, also whether it was the ARM64 Windows installer (a preview), the generic .deb or .rpm or your release's own, the container image or the Helm chart, and from 6.3.8 on the Ansible collection, the Kubernetes operator or the container on Proxmox VE. Behaviour differs between them.

Database backend. Built-in (SQL Server Compact), MySQL or MariaDB, Microsoft SQL Server, or PostgreSQL. On Linux only PostgreSQL and MySQL/MariaDB are accepted. The built-in database and SQL Server are refused by name there. Give the database server version too, not just the product.

Upgrade or fresh install. If it was an upgrade, say what it was upgraded from. Upgrades are in place and do not require stepping through intermediate versions, so if yours behaved otherwise that is itself the report. If you upgraded to 6.3.4 and a schema step stopped, give the step and the database's error number (for example 1071 or 1062 on MySQL or MariaDB). If you upgraded from 6.3.1 or 6.3.2, say whether the Control Panel was closed first. If the install is fresh, say so. It rules out half the causes immediately.

Diagnostics output. Run Monitoring & troubleshooting → Diagnostics in the Control Panel before posting and paste the result. It self-tests DNS resolution, database connectivity, port bindings and disk access, and it often answers the question on its own. See §16.4 of the documentation.

The relevant log lines. Logs live in the Logs directory on Windows and /var/log/hmailserver on Linux. Monitoring & troubleshooting → Logging controls what is written. Turn debug logging on only long enough to reproduce the fault, then turn it off. It is verbose and it will fill the disk. Paste the lines around the failure in a fenced code block. Not a screenshot. Ten lines of context beats a 40 MB attachment.

What you already tried, and what changed before it broke.

Redact before you paste. SMTP AUTH exchanges and database connection strings both land in the logs verbatim. AUTH PLAIN and AUTH LOGIN arguments are base64, which is an encoding, not encryption, and anyone reading the thread can decode them. Replace passwords, API keys and connection strings with placeholders. Keep hostnames and IP addresses where you can, because they are usually the useful part. This forum is public and indexed.

Downloads and release notes: https://www.progressiverobot.com/hmailserver-downloads/