.. because I cannot email you at the moment. (You don't need to have a @yahoo email address to be shackled to this bunch of incompetents either: if you're a customer of Sky or BT broadband, your @sky.com etc email address is handled by Yahoo too.)

Until last week, I could. But I have just changed the server that handles my email and Yahoo is blocking the new one.

The actual message Yahoo give when I try is:

421 4.7.1 [TS03] All messages from (your server) will be permanently deferred; Retrying will NOT succeed. See http://postmaster.yahoo.com/421-ts03.html

That URL doesn't go to the right place (you can get an idea of the quality of Yahoo already, can't you?) It should be help.yahoo.com/kb/mail/SLN3436.html, which says:

This message indicates Yahoo is seeing a high volume of emails from your IP address, which is a characteristic of unsolicited, bulk emailing. .. we do not disclose further information about our filtering practices.

One of the joys of running your own mail server is that is that you can see for certain how many a "high volume" is. In this case, it’s four. In a week. Instead of being unsolicited, all were replies to emails sent from Yahoo. And it's blocked them all.

Another page reckons that a 421 error means..

Resources temporarily unavailable. Please try again later: This means that the Yahoo Mail servers are temporarily too busy. Your server should try to resend the message after a few minutes.

.. which it does, and the server should. Except that they've just told you in the actual error message not to.

Elsewhere on the mess that’s Yahoo’s 'help' area is the advice to "secure your mail servers". Not that Yahoo would recognise a secure email server if they fell over one: hacked Yahoo accounts are the single biggest source of the spam that gets through the filters here (the victims are people I know, so their mail is treated differently to that from addresses I’ve never heard of).

The only things that changed between the old server and the new were (technical stuff coming up!) the IP address and the domain name of the server. Both before and afterwards, the IP address was not in any blacklist, the domain name of the server matched what you get if you look up its IP address (a simple test that most spambots fail), and the SPF ('sender permitted from') records for the domains of the email said 'yep, this IP address sends our email'. Since noticing this problem, I have even implemented Yahoo's half-baked DKIM system (another way of saying, 'yes, this server sends our mail') to no avail.

Yahoo, of course, don't make it easy to report a problem, but the page to complain about them blocking email you send is (currently) here. I've done that, even though it's aimed at people sending "bulk" email. Not a peep back and my mail is still blocked.

You're generously allowed to resubmit it after 48 hours, which I did after setting up DKIM and verifying that it works (via the useful check-auth at verifier.port25.com facility). Still nothing from Yahoo and my mail is still blocked.

So, at the moment I have no way of knowing when they will let me email you, even in reply to an email you send me.

You could complain to them about it – and please do, if you can find out how – but the basic cure is to use someone else for your email. Gmail is not perfect, but it is so much better than Yahoo.

Because who else are they blocking – including, as here, blocking replies to your emails – without either asking or telling you?