Hi,
Spacemail isn’t recommended as a mailing service for Discourse as it’s recommended for personal usage. It is very likely that they are going to block mass emails and/or it just simply won’t work. I use it too.
Try using Mailjet, combined with Spacemail, and simply use Mailjet for Discourse-emailing and leave Spacemail for personal use. You can have more than one mailing provider in DNS records.
Edit:
You can bypass it as @pangbo has suggested but it’s not a long-term solution.