When you will send emails to your recipients, then the inbox provider will check all the details of your email's sender.
Such as your subject line, your content, link present inside your email, your sending speed, the reputation of your IPs/domain, past behavior of your mailing, your interaction with the user (did you communicated in the past), did the sender is present in users contact list and many other factors.
And if that ISP will feel your content is spammy, your IP/Server/domain listed in the blacklist, the range of IPs are listed in the blacklist, the link present in your email content is listed, subject lines are considered to be spam, sending too many emails within a particular time range, then these can reject your email based on any of these single.
But not all the ISPs are that restrictive for such mailing.
Few ISPs are quite restrictive particularly as Outlook, Hotmail of Live IDs, to reach their inbox are too difficult now a day even for big senders.
Few ISPs are quite restrictive particularly as Outlook, Hotmail of Live IDs, to reach their inbox are too difficult now a day even for big senders.
As in the bounce message, you can see that they reject the email because of the few IPs (not your IPs specifically, but other IPs from the same network) already in their block list, hence they are not accepting it.
There any many email senders who did this kind of bulk mailing and OVH is a popular choice among them.
There are many types of the sender (few of them are the spammer too) so when most of them using their servers for these kinds of mailing, ISP (such as Outlook, Hotmail, Live) blocked the complete network instead of single IP.
There are many ISP and each and everyone have their own rules and regulation.
Unfortunately, this is something that can not be fixed since it will depend on those ISPs and which emails they want to accept and from which sender & from which network will be completely up to them.
Is there any problem with our configuration such as incorrect DNS or rDNS not matching etc?
There are many ISP and each and everyone have their own rules and regulation.
Unfortunately, this is something that can not be fixed since it will depend on those ISPs and which emails they want to accept and from which sender & from which network will be completely up to them.
Is there any problem with our configuration such as incorrect DNS or rDNS not matching etc?
In every emails server setup, we will do all the necessary configuration and setup proper DNS along with email encryption such as SPF, DKIM, DMARC, and IPs rotation with proper rDNS settings.
This is the basic requirement of any setup and we do this in each setup.
We can assure you that is not a problem with your configuration since we deliver the server setup after proper testing only that each of these encryption is passed or not.
What could be the problem then?
Microsoft itself mentioned many reasons for this as follows on their website.
Do I need to change the IPs or the server provider (instead of OVH some other VPS provider) for this?
We are used only a few VPS provider for our bulk mailing due to nature of this mailing and OVH is one of them which can provide the server and IPs without restrictions and quite lenient towards bulk mailing.
The majority of the server provider already disabled their mailing ports (ports 25, 587 and465 are already blocked due to spamming), so we have very few options left to use them as our mailing server.
There is no such problem with OVH but since there are many such users who use their server/IPs for bulk mailing (a few of them are spammers too) hence the Microsoft blocked the complete Network instead of blocking particular IP.
Even if you will change the IPs/VPS provider and take any other one, even there is a high probability that those IPs or network ranges will also be blocked
The majority of the server provider already disabled their mailing ports (ports 25, 587 and465 are already blocked due to spamming), so we have very few options left to use them as our mailing server.
There is no such problem with OVH but since there are many such users who use their server/IPs for bulk mailing (a few of them are spammers too) hence the Microsoft blocked the complete Network instead of blocking particular IP.
Even if you will change the IPs/VPS provider and take any other one, even there is a high probability that those IPs or network ranges will also be blocked
by default by Microsoft server.
It is only the OVH that was blocked by Microsoft but they blocked all those server providers who provided their server for bulk mailings such as Digital Ocean or even Amazon AWS servers too.
https://answers.microsoft.com/en-us/msoffice/forum/msoffice_o365admin-mso_other-mso_o365b/ndr-digitalocean-ips-blocked-by-office365-smtp/e0d97f59-83ac-450b-9f88-5d5cb83ec94e
https://answers.microsoft.com/en-us/outlook_com/forum/all/outlookcom-is-blocking-amazon-aws/466dd363-3f85-4bb4-af3f-70cfea447ef1
https://answers.microsoft.com/en-us/outlook_com/forum/osecurity-oother/email-server-being-blocked-by-outlookcom-despite/faa11dc9-c71c-436c-8ad0-72b575ab66aa
Can I request the removal of my IPs?
It is only the OVH that was blocked by Microsoft but they blocked all those server providers who provided their server for bulk mailings such as Digital Ocean or even Amazon AWS servers too.
https://answers.microsoft.com/en-us/msoffice/forum/msoffice_o365admin-mso_other-mso_o365b/ndr-digitalocean-ips-blocked-by-office365-smtp/e0d97f59-83ac-450b-9f88-5d5cb83ec94e
https://answers.microsoft.com/en-us/outlook_com/forum/all/outlookcom-is-blocking-amazon-aws/466dd363-3f85-4bb4-af3f-70cfea447ef1
https://answers.microsoft.com/en-us/outlook_com/forum/osecurity-oother/email-server-being-blocked-by-outlookcom-despite/faa11dc9-c71c-436c-8ad0-72b575ab66aa
Can I request the removal of my IPs?
There is not direct removal but you can follow these processes.
Although it will be hard (especially if you will send the non-optin mailing) to comply with all the things you can try the following thing.
1. Return Path Certification
1. Return Path Certification
2. Junk Email Reporting Program
Enroll at https://postmaster.live.com/snds/JMRP.aspx and typically start receiving feedback within as little as 72 hours
3. Smart Network Data Services
3. Smart Network Data Services
Learn more at https://sendersupport.olc.protection.outlook.com/snds/
Conclusion:
We don't want to disappoint or discourage you but these things are very common due to the nature of mailing that few of the ISPs will be rejected and bounce back your emails, so it will be suggested to focus on other ISPs which will still accept your emails and focus on your overall open rate.