How to fix 550 5.7.1 depends on the words after the code. The code says only that the receiving server refused your message on security or policy grounds, and the sentence that follows names the reason. The decoder above opens on the common meanings, and when you paste your own bounce it picks the one that matches.
How to read the result
The decoder finds every code in the text and looks for the mail service that wrote it. A Gmail bounce usually carries "gsmtp", and a Microsoft 365 one "protection.outlook.com". When it finds the service, that service's meaning comes first.
Two codes joined by an arrow mean one server passed on another server's refusal. The code after the arrow is the real reason.
Stock text such as "550 5.7.1 message rejected" or "command rejected" says little on its own. Look for the sentence that names a policy, a reputation, DMARC, or relaying.
Who has to act
Some 5.7.1 causes are not yours to fix, so sort by who acts first:
A recipient's policy, an admin rule, or a members-only group. Ask the recipient to have their admin allow your address. Microsoft's guide to 550 5.7.1 in Exchange Online says writing to a group that takes mail only from its members is a frequent cause.
DMARC. Make SPF or DKIM pass and align with your From domain for every service that sends as you.
A very low domain or IP reputation, or a block list. Send less, only to people who expect your mail, and ask for delisting where the bounce says how.
Relaying denied. Sign in to your outgoing server, or send through your own mail service's server.
What to do next
If the cause was the recipient's policy, you are done once their admin answers. If it was your reputation, the work goes on after this one message.
Addresses that do not exist are the easiest thing to take off a list before you send. Oneprofile can verify every address before a sequence sends, and it stops emailing an address once it hard bounces. To start from a checked list, find and verify emails for a whole list in one run.