Dealing with bulk Emailing might be one of the most challenging things an ISP has to deal with.  So many users bite on phishing attacks or are improperly protected from malware that spammers get a hold of their accounts and use it to send out spam to thousands of unlucky recipients. Sadly, these issues doesn't just impact the poor user whose account was compromised.  This impacts all users that share that mail system as a lot of the larger organizations have become weary from all of the spam complaints and so when they see bulk emailing coming from a mail server they just blacklist it and no user can send even non-bulk Email to that organization.  This is to some extent the lazy approach to dealing with spam but one of the few automated methods that works well and so this practice is unlikely to stop. Here's what you need to know if you wish to send an Email to a large number of recipients:

• Send your Email in batches of less than 50 recipients.
  
  
• Wait a few minutes between batches. 
  
  
• Make sure all of the recipients know you and welcome Email from you.  Some providers (e.g. AOL, Charter, Microsoft) will react to a small number of spam complaints by blacklisting a mail server. 

If our outgoing mail servers detect a batch of Email exceeding 50 recipients or too many outgoing Emails are sent in a short period of time your batch may be quarantined until a technician can approve the batch.  We just look at the subject and recipient list to make a determination that the batch is not from a compromised account.