Helping people with computers... one answer at a time.
Occasionally email will bounce with the error "timed out". We'll look at what that does, and does not, tell us, and what might be the real problem.
I get the following error message sending email to one specific address:
This is the Postfix program at host *****.
I'm sorry to have to inform you that the message returned
below could not be delivered to one or more destinations.
For further assistance, please send mail to postmaster
If you do so, please include this problem report. You can
delete your own text from the message returned below.
The Postfix program
<***@***.com>: conversation with ***.com[ 212.***.***.***] timed
out while sending RCPT TO
Why is this happening? It is only happing to the one address - all the other addresses are fine.
Unfortunately there's no standard here ... the failure could be for many different reasons.
The best I can offer here are some guesses as to what might be happening given the clues at hand.
First let's explain that error:
timed out while sending RCPT TO
When mail servers talk to each other using the SMTP protocol, or even when your email program sends mail using SMTP to a server, the machines involved begin what can best be described as a "conversation". The machine sending email sends a command, and the machine that would be accepting the message responds to each command. It's a very simple back and forth conversation, each machine responding to the other in turn until the message has been sent.
A "time out" is just one machine's failure to respond when it's supposed to. Machine one says something as part of the conversation and machine 2 responds with ... nothing. After a while machine 1 just gives up and reports the time out ("I waited too long and didn't get an answer") error.
Now, the RCPT command is the first command in the conversation that actually identifies who the mail is going to; it includes the email address. A message with several recipients will include several RCPT commands.
So what the error you're seeing indicates that the sending machine has said "this email is for so-and-so", and the accepting machine stopped responding.
Great. What does that mean?
We don't know. There's no standard for that behavior. However, here are a few guesses:
The email address you're sending to is invalid or doesn't exist. This is perhaps the most likely scenario of all. In order to thwart or at least slow down spammers who will blast email to thousands of addresses whether they exist or not many email servers have elected simply to stop responding when an invalid email address is detected. The reasoning is simple: the sender would get an error one way or another anyway, since the email address is invalid, but by forcing the sending server to wait for the time out is that sender happens to be a spam-bot, then it's being forcibly slowed in its effort to spam the world.
The email address you're sending to has some other kind of problem. This is less likely, but still possible. For example a too-full mailbox or other kind of temporary problem could lead to this. It's not supposed to, but it's possible.
Your sending mail server has been identified as a source of spam. Since you're indicating that other email addresses to this destination domain work, this is also less likely but given the complex nature of spam detection algorithms but it is still possible.
The recipient mail server simply has a problem. Impossible to say exactly what kind of problem, but a misconfiguration or other issue could result in this type of behavior.
So all that tells you very little.
Here's what I would do in your situation: first, triple check that the email address you are sending to is correct and really does exist - that's by far the most likely issue. If that all looks correct, then your only recourse is to contact the postmaster or person responsible for the receiving server and see if they can tell you why that single email address might be failing for you.