This is my current configuration:
I have two servers:
- Server A is accessible with domain.tld
- Server B is accesslble with sub.domain.tld
In the DNS record MX for domain.tld point to sub.domain.tld
My postfix server is installed on server B and send mails with the form email@example.com instead of firstname.lastname@example.org
Both servers run under Debian and for SSL web access let's encrypt is currently the one who manage certificates for domain.tld and sub.domain.tld, since I am not sure how wildcard are managed with let's encrypt both domains have there own certificates instead of sharing the same.
For my question:
Now that I managed to configure SPF, DKIM and DMARK for the postfix server, I wanted to setup TLS, but I am not sure which certificate is the correct one to use in this configuration. Since postfix is installed on Server B/sub.domain.tld, should I use the certificate for this domain? Or with my mails using the format of Server A/domain.tld should I copy and use the certificate from Server A instead?