Sending mail to external addresses compromised

As there have been several accounts compromised by a phishing attack our mail servers have been used to send SPAM to external mail addresses. This resulted in a blocking of mails sent from our external mail relays by the receiving mail servers.

At the moment our external mail relays are not listed on any DNS block lists anymore, there are still blockings from especially Yahoo.com and Microsoft-run mail servers like hotmail.com, outlook.com and live.com in effect.