the server returned the error the attempt to send data to the server (null) failed

well i had the same problem too but what i did is change and reverify the POP Info and then it worked!

I'm having this problem with 2 out of 6 PoP mail accounts in my Mac Mail client. (IMAP accounts work fine.) Sending continues to work, so the smtp servers are being successfully contacted. Receiving Pop mail is where the problem is, even though the accounts are set up correctly. I have had tech support from one of the mail service providers work with me to try to find the problem, without success. I have deleted and recreated one account, to get it working for a few minutes only. I used the mail account setup tool. But the rebuilt account worked ONE TIME only and will not work again. I have used the mail connection doctor to watch the bahaviour while tweaking the mail account settings for the other failing account. I get it working momentarily, and then it fails again, and then sometimes works. The message from both accounts is:

There may be a problem with the mail server or network. Verify the settings for account “xxxxxxxx” or try again.

The server returned the error: The attempt to send data to the server “(null)” failed.

Apple, you have a problem!

I have had this with BT Yahoo and outlook.

The only way I have found to solve it is to log into BT Yahoo using the web rather than Outlook. Log out again, then log in again, all on the web.

Outlook then works. No idea why.

Have just solved it this way, so I don't know how permanent it is.

cj

I just purchased an MacBook Air with Lion OS X v10.7.4 and have experienced the same problem as described by DavidPaterson.ca. Specifically, "Sending continues to work, so the smtp servers are being successfully contacted. Receiving Pop mail is where the problem is, even though the accounts are set up correctly." Has anyone seen a resolution to this problem?

Change your port on the incoming server to 995 (default is 110).

I have this same problem. Anyone come up with a solution?

Try and change your port on the incoming server to 995 or 587.

Helped here, hope it works for you as well.

Thanks, but port number was already 995. I migrated to the Air from a MacBook running Snow Leopard using the Time Capsule backup. Email client with is same account worked fine on MacBook running Snow Leopard, has never worked on Air running Lion. I triple checked the email preference settings to no avail. It's a mystery to me.

From your comment I take it that you have problems with receiving only.

What authentication methods are you using? (Password?)

Some additional info regarding the problem I describe above.

Prior to my post above, I wasted over 1/2 hour of the tech support person from my ISP trying to resolve the problem. He remote logged into my machine and tried various port, ssl, & authentication alternatives (as I had already tried) without success. Facts are:

1) it is only receive mail. Sending mail is not affected (though I don't actually send via that account any more).

2) authentication is password. (same pwd as for send mail)

3) ports 995 and 587 make no difference, nor does ssl on or off for any of the 3 port number alternatives

4) deleting the account and setting it up again results in a single successful reveive mail cycle, and then the problem recurs

I use 2 different ISPs, with 3 different accounts (URLs) all running POP mail. The problem occurs only with one ISP, the one with the single (infrequently used) account. That account is only used to receive and forward logs from my wireless router and network storage devices, so it is important to me, but is a receive-only account.

My solution has been to ask Google to solve my Apple problem. I have a GMail account set up in Mac Mail to collect the mail from the ISP POP account, and to forward that mail to Mac Mail. I have disabled the offending POP account in Mac Mail and I therefore get my logs mail via the GMail account inbox. I suppose I could just set my devices to forward logs to GMail, but I have other reasons to continue hoping for a solution.

As rwbrown indicates, the problem occurs with Lion and apparently not in Snow Leopard (and Leopard BTW). In my case, it arose shortly after the upgrade to Lion. Leads me to conclude some probability of insanity in Lion since:

-ISP's tech support couldn't solve it

-send mail is unaffected

-all port # & ssl alternatives have no affect

-account works on initial setup, then fails

-GMail works to receive from the ISP POP account without difficulty

DavidPaterson.ca has it exactly right. Sending works fine, receiving does not work. This is a problem in Lion, but not with prior versions of MacOS. It is hard for me to believe it is a problem with the ISP, as I have a couple of other accounts with the same ISP that work just fine on pre-Lion versions of MacOS. It appears to be the Lion O'S and email client, or in other words an Apple bug.

Just solved my problem. LIke David Paterson I was having problem receiving with one account and not others. The only difference in my account settings (other than the obvious Account Information) was in the Preferences > Advanced> "Use SSL" checkbox. the troublesome account was checked. When I unchecked the box, like other accounts, mail poured in.

Glad to hear you have solved it yowzas, but doesn't solve it for me. No combination of the port number changes suggested by __fb or setting SSL on or off makes a difference. Oddly, the same settings on my iPhone work fine.

Just bought a Mac Mini with ML installed and am having the same problem. I own my own domain and use Hostgator for hosting. Never had any problems on past Windows machines (this is my first Mac, running scared from Win8) or with Sparrow or Thunderbird which both work fine. Have tried the suggestions here to no avail. Too bad I will have to use one of the alternatives as I really like the Mac Mail interface.