How to ratelimit one email address for outgoing emails using rspamd? Is this available in ispconfig gui? If not, can share a config on how to achieve it?
That is not possible within ISPConfig itself right now. You have to use rspamd. The process is fairly well documented at https://docs.rspamd.com/modules/ratelimit/#user-defined-ratelimits
I have read the document in the link and do not understand it. Can share an example? Do I have to use the lua language?
Yes as described in the official documentation. I've never used this myself so i can't provide you with a working solution, but it shouldn't be to hard to figure out. We use quiet a few custom lua functions with rspamd, it is not that complex. Anyways you can use the built in rate limit module to implement rate limiting for all users.
