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A hostname lookup failure means that the server to which your email is being sent can't be found. This could be a temporary or permanent error.
I sent an email to someone who I normally email. But now I get a Mail Delivery (Undeliverable) Message back three hours after I sent the original email that says permanent fatal errors and then: Deferred Name Server: xxx.com host name lookup failure Message could not be delivered for three hours, message will be deleted from queue. And then, it says Action Failed Status: 4.4.7 and says Remote MTA DNS etc. I have tried this twice and gotten the same message back. What does this mean? Is it possible that they forgot to renew their domain name? Or could it be that I am now in their spam or something? Thanks for your help!
Three hours seems pretty quick for the server to give up on something like this.
There are a variety of possible problems here; some of which could last for longer than three hours and yet resolve themselves automatically.
In your case, it's unlikely that you are flagged as sending spam and I would certainly try to email the person again the next day, or perhaps using an alternate email address on a different domain.
Let's look at how this works and what some of the possible failures could be.
When you send email, one of the first steps that the sending server or mail program needs to do is determine what server on the Internet is designated to receive email for the domain.
It does this by querying the domain name system for what's called an "MX" or mail exchange record. This is very similar to the more common function of DNS where the computer might ask, "What is the IP address of this domain?" Instead, the question is more like, "To what server should I send email that is destined for this domain?"
For example, if email is destined for email@example.com, the sending email server or email program will ask, "What is the server to which I should send email destined for email addresses on randomisp.com?" DNS will respond with either another domain name, perhaps something like mail.randomisp.com, or a "not found". When DNS responds with a different name, this allows mail for a domain to be handled on a completely different server than the server hosting the domain's website.
In this case, "not found" is actually not an error, per se. The protocol is that if there is no mail exchange record for a domain, then the domain itself is used. So, if in response to our query for randomisp.com, the response had been "not found", then mail would be sent to the same server hosting the randomisp.com website itself.
In your case, the error indicates that either the server specified by the mail exchange record can't be found (i.e. mail.randomisp.com can't be found, to use my example) or the mail exchange record itself did not exist and that the domain name used in the email address was also not found (randomisp.com doesn't exist).
DNS can fail. By that, I mean that it is possible for DNS lookups to fail for a variety of reasons.
Some of those reasons are permanent. Perhaps the domain name has, as you theorize, not been renewed. A quick review of the domain registration using a whois service would tell you if this was the case. Perhaps the company has gone out of business. One of my first checks would be to see if you can access the website for the email service to which you are attempting to send email.
Failures can also be temporary. For example, DNS servers can sometimes go offline; they can crash or have other problems. Perhaps the data center used by the service has been cut off from the Internet for some reason. Typically, these problems are resolved relatively quickly, but for some domains and servers, that may take a while longer. That's kind of why I'm surprised that your sending server gave up after only three hours.
The entire mail system is actually built to generally tolerate delays of multiple days, not simply multiple hours.
In a case like this, it's difficult to know exactly what next steps to take. I'd try to go to the website of the email provider and perhaps check the DNS records and the domain registration for the provider. Assuming that all of it looked as it should, I would suspect a transient error.
But there's no way to know for sure.
As I said earlier, you're best bet is two-fold:
Try sending email again in a day or two. Almost all failures of this sort that are truly transitory are going to be resolved within that time frame. (If it truly is a transient problem and it's not fixed within a couple of days, you might suggest to your recipient when you finally make contact again that they find another provider.)
Try contacting your recipient some other way. Typically, that means using a different email address using a different email provider or giving them a phone call, and IM, or something else that doesn't rely on the domain of their email provider to work.
And for the record, I would also check to make sure that you've actually typed in the correct email address. A typo in the domain name could cause exactly this kind of behavior.
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