one SMTP node to pass mails to underlaying SMTP servers (proxy smtp)

Question

I am having an issue, that I require a stable public SMTP endpoint (mySmtpPublicNode). This endpoint should relay/route the emails to the corresponding mailservers, which regularly change (myInternalSmtpServerXY)

SMTP-client <--> mySmtpPublicNode <--> myInternalSmtpServer01
                              ^
                              +------> myInternalSmtpServer02
                              |
                              +------> myInternalSmtpServer03
                              |
                              +------> ....
                              |
                              +------> myInternalSmtpServer99

The myInternalSmtpServer[01-99] are all having their own TLD mail-domains. I require that my mySmtpPublicNode can accept all mail from all myInternalSmtpServer[01-99] and also has a "sticky" handling of the underlying domains to the corresponding myInternalSmtpServer[01-99] server.

My restrictions:

• I can only define stable DNS records for the myInternalSmtpServer[01-99] hosts
• I cannot define stable IP addresses
• myInternalSmtpServer[01-99] are mostly postfix or sendmail
• mail-domains may change from myInternalSmtpServer[01] to e.g. myInternalSmtpServer[22] during the day
• all the above nodes, except the client, are in a controlled environment and I have full (root) access to the servers.

How to solve this with Linux (or docker)?

How to create a mapping these mail domain examples on the mySmtpPublicNode?

mail-domain	internal SMTP Server
example.com	myInternalSmtpServer01
mysuperduper.com	myInternalSmtpServer03
...	myInternalSmtpServer[01-99]

How to automatically update the mapping?

• the mail-domains may change from myInternalSmtpServerXY to myInternalSmtpServerAB depending on IO, CPU or Disk requirements
• so the mapping can change during a day, if the mail domain gets migrated