But is it our fault?

Recently we’ve seen an increase in the number of customers who have contacted us with problems sending email. When connecting to our mail servers they have had an error trying to send.

On checking mail server logs we’ve discovered that the reason they are having problems is that our mail servers have rejected the connection. On checking the connecting IP address it seems their IP address is blacklisted at various RBLs that track IP addresses used for spamming.