docs: add ADR mail server

docs/ADR/003-mailserver.md

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+# 003-mailserver
+
+## Status
+
+In progress
+
+## Context
+
+Gandi free email will become a pay service in 2 month.
+
+In this condition it will be interesting to  study selfhosted mail solution.
+
+### domain name
+
+do I take advantage of this to change domaine name:
+
+Pro:
+
+- could test more easy
+- could redirect old domain name to new one untile end of gandi domain (2026)
+- get a more "normal" extention
+
+con:
+
+- need to progresively update every personal account
+
+### software choose
+
+mail server will run in nomad cluster.
+
+docker-mailserver -> 1 container
+mailu
+
+## Decision
+
+we will switch to another domain name on "https://www.bookmyname.com/": ducamps.eu""
+docker-mailserver will be more easier to configure because only one container to migrate to nomad
+
+## Consequences