If it really is just 100-150 old topics, then an nginx map makes even more sense.
I can see it from your point of view, that it's easy for NodeBB to do, although I urge you to consider that nginx is very good at what it does, and so having nginx check for an old url to redirect is quite simple.
For example, if the old forum had topics using this url: /t/<topic id>, then you could write a location block that only responded to that folder, and applied the map to redirect the user based on those topic IDs.
@pitaj Thanks a lot : that really makes much more sense now.
I allready had deployed mailgun as a service through the use of an API key and that's what i was looking for !
Now that my domain has been created in the mailgun admin pannel and verified through the propagation of specific DNS records, i can confirm that the SMTP credentials are working and that the mails are being sent !