Namecheap.com DNS pointing "@" and "www" (A Records) to the ip address of my webserver
Namecheap.com DNS pointing "mail" (MXE) to the ip address of my mailserver
I have an SSL certificate for mail.domain1.com - all other domains access their email through mail.domain1.com
*) If you want to provide webmail on iRedMail server, you should point both A and MX records of "mail" to mail server.
*) If you don't want to provide webmail on mail server, you can setup webmail on webserver, and config webmail to use IMAP/SMTP server on mail server.
About iRedAdmin (httpS://domain.com/iredadmin/):
*) you can announce its URL to be httpS://[mail_server]/iredadmin/, then no httpS://domain.com/iredadmin/ at all.
*) Or, add a simple HTML file under httpS://domain.com/iredadmin/ to redirect all access to httpS://[mail_server]/iredadmin/.
*) Or, run iRedAdmin on webserver, but configure it to connect SQL/LDAP servers on mailserver. You can define them in iRedAdmin config file "settings.ini" (/usr/share/apache2/iredadmin/settings.ini).