If you are warming up an email address or sending outreach messages under a given domain and have an email bounce back on you with an error code similar to the one posted below.
The Response From The Remote Server Was:
550 High Probability of Spam
Then this is an indication that there is some type of reputation or DNS issue occurring under your sender domain.
Why Am I Only Getting This Response Back?
When you attempt to send an email from one mail server to another, there are a few processes that the receiving mail server will perform in order to determine the trustworthiness of the incoming message. In short, these mail servers will check
The header section of your email
The reputation of your domain and mail servers' IP address
The message content of your email.
When checking the header of your email, mail servers are specifically looking to make sure that your emails are signed correctly, and confirming that this email is coming from the mail server specified (verifying that this is not a spoofed email).
When a mail server determines the reputation of an email address, they have the option of either allowing the email to continue on and be sent over to the designated address as normal, or to deny this message if they feel that it is potentially spam or dangerous to accept.
If a message gets denied due to the mail server determining that your email appears as spam, this is when you will receive a bounce back stating the "550 High Probability of Spam" error.
What Can I Do To Prevent This In The Future?
If you are receiving a high amount of bouncebacks due to an SMTP 550 error, you should immediately check to make sure you have SPF, DKIM, and DMARC properly set up under your domain. Occasionally some spam filters/mail servers are a bit more strict than others, and can simply reject a message and label it as spam if the sender domain does not have these security protocols in place. In general, having these protocols in place will only help with deliverability, so ensure that everything is properly setup.
You should also check if your domain is on any blacklists, as having your sender IP exist under any blacklist can be enough cause for a spam filter to outright reject your mail due to any past activity that originally could have landed you on such a list.
If you have confirmed that your sender domain has all security protocols correctly implemented, as well as checked to see if your domain exists under any blacklists, the most likely culprit for receiving this error would be due to having a poor sender reputation. There are various other reasons as to why this error will be thrown back with a piece of rejected mail, but in most circumstances, it would be due to the current standing reputation of a domain's IP.
The most effective way of resolving this issue in the future would be to build your sender reputation back up through Inbox Warming and pausing all outside sender activity you are utilizing this sender domain for. If you are in the middle of an active outreach campaign, we would advise to immediately swap out the problematic inboxes under a sender domain with another sender domain you control if possible.
For building back up the reputation of a sender domain suffering a 550 error, we would advise connecting all problematic inboxes to the platform, and keeping them running at a low sender volume for around 45-90 days total, while paying attention to spam rates on the platform and routinely checking into the inbox itself to see if any emails are getting rejected due to this SMTP 550 error. Over time, this should build up your domain reputation and make it much less likely to experience this bounce in the future.