Feature: allow to configure preference for IPv4 or IPv6 connectivity for mail domain

[timvisee]

I would like to request the following feature:

It would be awesome if you could configure whether Rainloop attempts to resolve an IPv4 or an IPv6 address for a given IMAP/SMTP domain that is configured in the Domains section of the administration panel.

Some cloud providers block outbound SMTP connections by default (such as Digital Ocean [screenshot]) on IPv6 specifically, to prevent big IPv6 ranges from being blocked by other mail providers as spammers tend to abuse these virtual servers.

On such servers (including one of mine), a configured SMTP domain may resolve an IPv6 address which will result in an error causing these packets to drop. Using an explicit IPv4 address works fine, but it is undesired as it might change in the future.

Curl, which is used for SMPT connectivity by Rainloop, provides options to explicitly resolve an IPv4 or IPv6 address as described here:
https://curl.haxx.se/libcurl/c/CURLOPT_IPRESOLVE.html

A simple dropdown with the options [Unspecified, Resolve IPv4, Resolve IPv6] in the domain dialog for configuring this would be enough.

What do others think?