I am on the board of a small sports club. None of us is a professional web master or administrator. A hobbyist club member works as our webmaster in his free time, and none of us knows what to make of the current situation. We have a forum, which requires registration. Our system automatically answers with a please click this link to confirm registration mail. In the last weeks, we got bombarded with both e-mail spam and attempts to register nonsense accounts on the forum. It seems that these were so many that our automated mails fail. Is there cause for concern here? Can we get blacklisted somewhere because our server is spewing out so many mails in reaction to the illegal registration attempts? Should we be taking action, and what can we do?This is the mail system at host [redacted; it is always the same].I'm sorry to have to inform you that your message could not be  delivered to one or more recipients. It's attached below.For further assistance, please send mail to postmaster.If you do so, please include this problem report. You can delete your  own text from the attached returned message.The mail system: host mail.mailsdfsdf.com[5.61.38.11] refused  to talk      to me: 421 Too many concurrent SMTP connections; please try again later.Reporting-MTA: dns; [redacted] X-Postfix-Queue-ID:  97EA5723478 X-Postfix-Sender: rfc822; [our mail address] Arrival-Date:  Wed, 28 Oct 2015 12:56:04 +0100 (CET)Final-Recipient: rfc822; urukef@mailsdfsdf.com Action: failed Status:  4.0.0 Remote-MTA: dns; mail.mailsdfsdf.com Diagnostic-Code: smtp; 421 Too many concurrent SMTP connections; please try      again later.
Could we get blacklisted somewhere when our register form is abused?
botattack;blacklist