It is a message from Apple Mail but it is not an Apple Mail issue that you can fix by adjusting something on your Mac or in Apple Mail. The Apple Mail program is simply reporting that the server at wctel.net is telling you that the server from your email provider blocked the message in one instance because of "too many sent messages" and in the other because the server detected "spam content."
The one reporting "spam content" appears to have many email addressees in the bcc which is one of the hallmarks of spam. That said, your service provider seems to be a bit touchy about using its service. It limits your Sent email messages (you proved that by reducing the number to fix an issue) and it seems a bit restrictive of the "spam detection."
The only limit I remember is 50 messages per hour, regardless, I had not even come close to any of the limits they mentioned.
I think you need to explore these limits more with your email provider. Or simply find a new email provider -- I'm sure your GMAIL imposes no such restrictions, for instance, and you appear to be using the same Apple Mail there as well. Or simply use GMAIL instead.
You can configure a Rule in Apple Mail (see Settings or Preferences and select the Rules tab) to delete all Sent message older than something that you specify. But there is no reason you should have to do that with POP. And those restrictions on number of emails sent or number of emails sent per hour are something that would, for me, make an email provider unacceptable, I would use a different provider.
