If email providers send you an email to notify you that an email address is invalid and request that you stop sending more emails than its a violation of their terms and of the law to keep sending more emails to the provider.
It's not much different from sending emails to someone who has unsubscribed from your list.
The provider has let you know that you should stop sending emails. If you ignore that and keep sending more emails then you are sending spam. Mind that you are not just sending an email but are sending bulk mail.
Also mind that you will probably not find out when you are blacklisted as a spammer. Email providers have a number of severities in blacklisting. This depends upon the number of violations a provider finds within a time span. If severe enough the provider will put your domain on a shared blacklist. If multiple providers find violations then your severity rises and more and more providers will block your mail and more spam filters will stop your email from arriving.
Often you will not find out that you are blacklisted until members start complaining.
The main problem with all of this is that the xenforo emailing method lacks all tools to alert you of problems and managing these problems.