I performed a handful of checks to see:
- Does the domain have an A record? No. We're getting a server failure response or server not found response.
- Does the domain have an MX record? No. We're getting a server failure response or server not found response.
- Does that MX or A record answer as a mail server when you try "telnet (hostname) 25"? Doesn't matter, can't find either.
- Search the web -- what do I find? Two things: First, Matt Vernhout blogged about this domain shutting down back in 2009. Second, here's information from an AT&T message board suggesting maybe the domain still worked in 2016.
According to this timeline, Windstream seems to have indeed picked up much of the Alltel user base at some point in the past through M&A, but AT&T may have later acquired some of the Windstream properties back later? Confusing. But regardless, the domain certainly seems dead today. There's no point in sending to it, and if you've got alltel.net subscribers on your list, then something is funny. Is your list old? Are you sure it's all people who recently opted-in to receive email from you?