Hello,
I actually ran into a similar problem last year after a mail server
migration. Here's what I documented back then in my blog:
"Deutsche Telekom, respectively T-Online, by default blocks IP addresses
that haven’t been used for sending e-mails to their servers for a
certain amount of time. You can test if you are blocked by connecting to
their mail server on port 25 – if the blocking is active, the connection
will get immediately dropped with an 5xx error message, that lists a
contact address to request unblocking from. To test, run the following
command from your mail server:
telnet [-b floating IPv4] mx00.t-online.de 25
When I ran into the problem, they were quite fast in reacting and
removed the blocking in about an hour. However, as per their use policy,
they require the mail server’s main domain to have a proper imprint. In
other words, if your mail server’s hostname is mail.mydomain.tld, you
must place a proper imprint at mydomain.tld."
I just checked on a machine I operate, and they still can deliver to the
Telekom MX'es. So right now I would guess that only the error message
has changed, without a change in the policy.
The imprint thing is probably something very specific German, nearly
everyone needs an imprint here.
I don't comment on whether all this is sensible or not, but I hope the
above helps a bit those of you who run into problems.
Florian
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