What is internal bounce?

There are 3 types of bonce categories as follows.

Hard Bounces - A hard bounce means the email does not exist anymore on the target server, or it has been blocked/disabled or the target server does not exist, etc.

Soft Bounce - Unlike hard bounces, soft bounces can happen for reasons like server/account temporarily unavailable, not enough disk space to store the email on the server, or the receiving server is too busy, etc.

Internal Bounce - 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, etc. (Related to your email content or your IP/domain's reputation)

In email delivery the ISP (such as Gmail, Yahoo, Outlook etc) they will check each and every option starting from your subject line, your content, link present inside your email, your sending speed, the reputation of your IPs/domain.

Few ISPs are quite restrictive for excepting any emails such as Outlook, Hotmail of Live IDs.

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 sender 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 which 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.

These things are very common due to nature of this kind of mailing that few of the ISPs will be rejected and bounce back your emails, so it will be suggested to focus other ISPs which will still accept your emails and focus on your overall open rate.

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