If you can temporarily disable TLS between your server and mailersend, that’ll let you inspect the actual traffic on the wire and it’ll show you whether Discourse is sending the spaces or not, setting this question once and for all.
If you can’t, you could try MITMing the traffic, but that’s more complicated.
If neither of the above work, for this case I would configure a local postfix, but not configure it for direct delivery, rather have it send its email to your mailersend account the same way Discourse does.
That way you can have Discourse send via either method and you can inspect the mail in the postfix queue before it goes out.