Are you affected?
Are you the problem?
If you are sending out bulk emails (emails with lots of addresses in the header) then your mail may be the reason why a server is blacklisted. This area is a major problem because most mail is junk mail. Some speculate that as much as 80% of mail is junk mail or spam. Service providers use various methods of identifying rogue sources of mail, these systems are automated and do not always get it right.
You may be aware of BT related problems on a number of occasions over the past year. BT are now more helpfully providing a link in their mail rejection notice to some guidance on best practice for creating bulk emails.
You can read about it here: BT Best Practices for Bulk Mail.
If your organisation sends out newsletters to people by sending them to lots of people on the to:/ cc:/ or bcc: lines then please read the article and check your practices.
Additionally the general guidance is no more than 250 emails are sent per hour. I recommend use a lower number like 200. Noting that a single email to 10 recipients is effectively 10 emails and not 1.
You should also avoid having too many people per email. Probably best to stick to 15 or less recipients. Noting that if AOL see the same message going to lots of people, it will suspect that the message is junk mail. Likewise with BT. Most of the large service providers use tools within their network to assess mails and establish a rating for the mail that relates to the likelihood of the mail being a junk mail or spam.
It may be better to seek a bulk mailing service if you need to send out a lot of emails to people with the same message. Or use a service built into your website like mailpoet where each mail is individually sent to recipient and can be timed.