fix(server): normalize mail server name (#14564)

fix #14562 
fix #14226 
fix #14192


#### PR Dependency Tree


* **PR #14564** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* SMTP and fallback SMTP name now default to empty and will use the
system hostname when not set.
* HELO hostname resolution includes stricter normalization/validation
for more reliable mail handshakes.

* **Documentation**
* Updated admin and config descriptions to explain hostname/HELO
behavior and fallback.

* **Tests**
* Added tests covering hostname normalization and rejection of invalid
HELO values.

* **Chores**
  * Updated example env and ignore rules.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.gitignore

@@ -48,6 +48,7 @@ testem.log
 /typings
 tsconfig.tsbuildinfo
 .context
+/*.md
 
 # System Files
 .DS_Store