Woah; Now I understand why so much user end by not using KMail (if you are curious look at KDE forum and Reddit for KMail issue) Basically @girish you were on the right track simply I had to go a little bit deeper instead of looking under identities (because, of course, my Identity was well configured) the issue is located under the sending configuration where you have to declare the mail email (not the alias even if my test is not with the alias.) Anyway thanks for your help!

Also a useful thread on the subject of ports and autoconfig/autodiscover: https://forum.cloudron.io/topic/1296/add-dns-dav-registring/4?_=1595212232880

@christiaan oh super cool! Never heard of JMAP before, that looks great.