Hello,
I'm rather new to this list, so I hope it's okay to jump in directly in
this discussion and share some thoughts. :-)
I just recently faced a very similar problem with one of the new IPs I
got assigned. Likely, the IP was in use by someone else previously or
there are spammy neighbours (not surprising when it's a big provider
with some customer turnaround). I got all deblocking done in the various
blocklists, the status was clean everywhere, including SNDS, but still,
Microsoft 365 mailboxes (*.protection.outlook.com) blocked it.
It seems there are various independent blocklists - one set is covering
consumer Outlook/Live/Hotmail, the other set is covering Microsoft 365.
SNDS seems to only query the former one, and the Outlook.com web form
only unblocks on these domains, so neither of that helps for Microsoft
365 mailboxes.
For Microsoft 365, there seem to be various levels of blocking.
Sometimes, you can unblock yourself at sender.office.com, but I hit the
infamous "Write to delist@" message. I managed to create a ticket there,
they asked for some details, but then radio silence, all follow-up mails
ignored, for over a month now. Various ways of contacting them went
without any outcome, and I sense that on their end there is also some
confusion as to the existing blocklists - I got repeatedly told the IP
is not listed (which is what sender.office.com told me too), or to use
the Outlook.com form, that obviously didn't help for my problem at hand.
In the end, via way one of the communication channels I tried, I somehow
managed to get a supporter who was really engaged and managed to helped
me out, but I must admit that the whole process was not straightforward
and took me a month in the end. At least when people provide full
contact data/imprint and proof that the IP was freshly assigned, some
feedback would be good to not be left in the dark.
Problematic is also that there is no actual way to test if your new IP
is blocked before using it, apart from sending to an actual e-mail
address hosted there.
Hope to help,
Florian
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