Yesterday we updated our SMTP servers so that they deliver mail using a pool of 30 IP addresses. Previously outgoing mail from all of our SMTP servers was NAT’d through a single outgoing IP address. Using a single IP however left our system somewhat vulnerable, because if that IP address were to become temporarily blacklisted by an anti-spam group, it could affect the deliverability of all customer mail. We have dealt with this situation in the past by switching to a new unused IP temporarily while addressing the problem. However you can’t do that anymore because SenderBase.org and TrustedSource.org will see a huge mail volume spike, which their partners use to further block mail.
Now with the use of a pool of outgoing IP addresses, if an IP becomes temporarily blacklisted, we can pull the affected IP address out of use while we deal with the customer who got us blacklisted.
Fyi, our IPs have only been blacklisted three times during the six years we have been in business. In both cases it was resolved with within hours.
Webmail.us customers who have SPF records for their domains should continue to
use the following value in order to always have the updated IPs
automatically included in your domain’s SPF record:
example.com IN TXT "v=spf1 include:emailsrvr.com -all"