==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 2020043001
- Deployed with iRedMail Easy or the downloadable installer? Easy
- Linux/BSD distribution name and version: Ubuntu 18.0.4 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? Yes
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

Since upgrading using the Easy installer the global SMTP relay was disabled - presume this is because main.cf was completely replaced.  So I thought I would just enter the server name at the Relay tab for each Domain, which I entered as per the attached screenshot.  However, after doing this, emails sent from this domain started bouncing with the error:

<user@icloud.com>: host mail.smtp2go.com[139.162.6.108] said: 550
   relay access denied - please authenticate (in reply to RCPT TO command)

The username and password is still stored in /etc/postfix/sasl_password and I also ran:
postmap hash:/etc/postfix/sasl_password
service postfix restart

but to no avail, as it doesn't appear to be picking up the authentication details from there.

How do we enter the authentication username and password in iRedAdmin Pro?

If this is not possible, what is the best practice to setup a global relay server.  In the docs I read that main.cf should be modified by editing /opt/iredmail/custom/postfix/custom.sh

Are you able to advise what should be added to 'custom.sh' to set the relay server as:
mail.smtp2go.com:2525

Thanks a lot,
Stephen