@oculos I’ve now managed to answer both of these questions myself, you can see the results.
tl;dr You can add the nodes to your existing domain certificate as additional SAN hosts, and you can adjust your connection strategies based on node_hostname.
Rate Limits - Let's Encrypt
I already know I am at my 7 day rate limit on the “same set of identifiers”.
Even though you are limited to 5 certs in 7 days against “identical identifiers” (e.g. the same 5 SAN hosts you start with on the same “registered domain”) if you add more SAN to the same cert, you are treated as a new order, subject to the 50 new orders in 7 days limit instead.
Therefore I am re…
mx1.example.com
Add Source IP = 10.0.2.100 + EHLO Hostname = mx2.example.com
Add Source IP = 10.0.3.100 + EHLO Hostname = mx3.example.com
Mail delivery log, which is clearly indicating the IP it is attempting to u…
Hope that helps.