AT&T
AT&T is the world's largest telecommunications company, headquartered in Texas. It offers free email accounts for past customers at sbcglobal.net and bellsouth.net, and new and non-customers can register an att.net or currently.com free email account. AT&T provides a detailed Postmaster page with contact options for investigating false-positive spam verdicts.
att.net, sbcglobal.net, swbell.net, bellsouth.net, pacbell.net, prodigy.com
3 responses across 1 reply codes
SMTP 550
3 responses
5.2.1
AT&T says the addressee is unknown. Treat it as a hard bounce and remove the address.
5.2.1-2
AT&T rejected this as an invalid address. Treat it as a hard bounce and remove it.
5.7.1
AT&T requires a valid sender domain and reverse DNS on the sending IP. Fix the PTR record, wait for propagation, then resend.
Response catalog adapted from the MIT-licensed SMTP Field Manual by ActiveCampaign / Postmark.