Shared IP

What it means

M3AAWG defines a shared IP as one that many different senders, brands or ESP customers mail from, usually at the same time, identified with the ESP that owns it rather than with any one brand. AWS SES sends new accounts from IP addresses shared with other SES users by default.

Shared IPs suit smaller senders. M3AAWG explains that combining several senders' volume keeps the overall rate steady, which IP reputation is sensitive to, that one sender's mistakes are diluted, and that shared environments cost less. The trade-off is that you share the consequences of other senders' practices, so the ESP's vetting matters.

M3AAWG recommends that each sender on a shared IP sign with DKIM using its own domain, so providers can tell the senders apart and the sender can carry its reputation elsewhere.