I've rechecked every SSL config and found in dovecot.conf the following

#ssl_ca = </path/to/ca
ssl_cert = </etc/pki/tls/certs/iRedMail.crt
ssl_key = </etc/pki/tls/private/iRedMail.key

Replaced according to the manual to

ssl_ca = </etc/pki/tls/private/iRedMail.key

But this not helped at all - the server can not be found. Which path (port) is trying to open Outlook when checking this type of connection?

Change this back to default setting and create symbolic links as follows:
mv /etc/pki/tls/certs/iRedMail.crt{,.bak}       # Backup. Rename iRedMail.crt to iRedMail.crt.bak
mv /etc/pki/tls/private/iRedMail.key{,.bak}     # Backup. Rename iRedMail.key to iRedMail.key.bak
ln -s /etc/letsencrypt/live/mail.mydomain.com/fullchain.pem /etc/pki/tls/certs/iRedMail.crt
ln -s /etc/letsencrypt/live/mail.mydomain.com/privkey.pem /etc/pki/tls/private/iRedMail.key

Here is my current SSL related configuration

[root@ired nginx]# grep -r ssl /etc/dovecot/dovecot.conf /etc/nginx/templates/ssl.tmpl
/etc/dovecot/dovecot.conf:ssl_protocols = !SSLv2 !SSLv3
/etc/dovecot/dovecot.conf:ssl = required
/etc/dovecot/dovecot.conf:verbose_ssl = no
/etc/dovecot/dovecot.conf:ssl_ca = </etc/pki/tls/certs/iRedMail.crt
/etc/dovecot/dovecot.conf:ssl_cert = </etc/pki/tls/certs/iRedMail.crt
/etc/dovecot/dovecot.conf:ssl_key = </etc/pki/tls/private/iRedMail.key
/etc/dovecot/dovecot.conf:ssl_cipher_list = ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+3DES:!aNULL:!MD5
/etc/dovecot/dovecot.conf:ssl_prefer_server_ciphers = yes
/etc/dovecot/dovecot.conf:# With disable_plaintext_auth=yes AND ssl=required, STARTTLS is mandatory.
/etc/dovecot/dovecot.conf:# Set disable_plaintext_auth=no AND ssl=yes to allow plain password transmitted
/etc/dovecot/dovecot.conf:    #    ssl = yes
/etc/dovecot/dovecot.conf:    #    ssl = yes
/etc/nginx/templates/ssl.tmpl:ssl on;
/etc/nginx/templates/ssl.tmpl:ssl_protocols TLSv1.2;
/etc/nginx/templates/ssl.tmpl:ssl_ciphers ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+3DES:!aNULL:!MD5;
/etc/nginx/templates/ssl.tmpl:ssl_prefer_server_ciphers on;
/etc/nginx/templates/ssl.tmpl:ssl_dhparam /etc/pki/tls/dh2048_param.pem;
/etc/nginx/templates/ssl.tmpl:# To use your own ssl cert (e.g. LetsEncrypt), please create symbol link to
/etc/nginx/templates/ssl.tmpl:# ssl cert/key used below, so that we can manage this config file with Ansible.
/etc/nginx/templates/ssl.tmpl:ssl_certificate /etc/pki/tls/certs/iRedMail.crt;
/etc/nginx/templates/ssl.tmpl:ssl_certificate_key /etc/pki/tls/private/iRedMail.key;

And the files (links)

[root@ired nginx]# ls -l /etc/pki/tls/certs/iRedMail.crt /etc/pki/tls/certs/iRedMail.crt /etc/pki/tls/private/iRedMail.key /etc/pki/tls/certs/iRedMail.crt /etc/pki/tls/private/iRedMail.key
lrwxrwxrwx 1 root root 49 Dec 30 10:55 /etc/pki/tls/certs/iRedMail.crt -> /etc/letsencrypt/live/ired.X.com/fullchain.pem
lrwxrwxrwx 1 root root 49 Dec 30 10:55 /etc/pki/tls/certs/iRedMail.crt -> /etc/letsencrypt/live/ired.X.com/fullchain.pem
lrwxrwxrwx 1 root root 49 Dec 30 10:55 /etc/pki/tls/certs/iRedMail.crt -> /etc/letsencrypt/live/ired.X.com/fullchain.pem
lrwxrwxrwx 1 root root 47 Dec 25 17:59 /etc/pki/tls/private/iRedMail.key -> /etc/letsencrypt/live/ired.X.com/privkey.pem
lrwxrwxrwx 1 root root 47 Dec 25 17:59 /etc/pki/tls/private/iRedMail.key -> /etc/letsencrypt/live/ired.X.com/privkey.pem

Your Nginx is set to only accept TLS1.2 connections, yet your outlook is negotiating handshake with weaker TLS/SSL protocol. Try allowing TLS 1.1 / 1.0 in Nginx to test.

Problem resolved now?