Up: E-mail

E-mail Forwarding Filtering

Protecting our customers from abusive e-mail is important to us. That is why we work hard to protect you from spam and viruses when using our E-mail Forwarding service.

Here are a list of filters that we currently apply to every single e-mail that comes through our E-mail Forwarding service. Most of these filters happen at SMTP time, meaning that the sender should receive immediate feedback if the e-mail is blocked.


  1. When a sending e-mail server is flagged as abusive by any pair Networks servers, they are blocked from being able to access the e-mail forwarding servers.
  2. Greylisting: The first time an e-mail server contacts us, the e-mail is temporarily declined with a message, 450 Service temporarily unavailable. Legitimate e-mail servers should try to redeliver the e-mail after 20-60 minutes, but we allow redelivery for up to 18 hours. When the redelivery is made, we mark the e-mail server as valid. For the next 30 days, other e-mails from that server will not be delayed.
  3. Real-time Blackhole List (RBL) Checking: We check the sending e-mail server against RBL lists from Invaluement.  If they flag a server, we block e-mail from it.
  4. Sender Policy Framework (SPF) Checking: If the sender’s domain has an SPF record, we check if the e-mail originated at one of the servers specified in the records. If the record specifies soft failures with “~all”, we add a Received-SPF header to the message. The header indicates the message was scanned and notes whether it passed the SPF check. If the record specifies hard failures with “-all”, any failed e-mail is blocked.
  5. Virus Scanning: We check the e-mail in Clam AV to see if there is a virus or malicious content. E-mail that has a virus will be bounced with an error, 521 Our system detected malicious content.
  6. Spam Checking: We check for spam using Spam Assassin. First we check against block lists from Spamhaus, Passive Spam, and Invaluement. If the message is flagged by any two of those lists, the e-mail will be blocked with an error, 521 Our system detected spam or links in a DNS blocklist.
    After the first check, we do a full check in Spam Assassin. This check will not block the e-mail. Instead it adds X-Spam headers to the e-mail and then forwards the e-mail. Your e-mail program can be set up to filter based on the the headers.

No system is perfect and we can’t guarantee that you’ll never see spam. We are continually refining our filtering process to give you the best possible experience. The list above represents the current filtering in place, and it is subject to change.

In order to avoid abuse of victims of “Joe Job” spam (spam with a forged sender), our system will typically not send mailer daemon bounce message back to the From or envelope sender address of emails.

Please note that most mail services do not distinguish between services that generate spam and the services that merely forward it. In order to protect the reputation of pair Networks in the global email community, we are unable to disable filtering when using E-mail Forwarding. We are unable to alter filtering settings for individual customers.