Email Message Rejected - Need help deciphering this.

brothertruffle880
brothertruffle880 used Ask the Experts™
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I sent a message to a friend and it got rejected.  Here's the error I received.
Please help me decipher!
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Unfortunately, that's a pretty vague error message you got back.

Are you sending from your own domain?  If so, do you have SPF and DKIM set up correctly?  If not, that may be the problem.
Top Expert 2016

Commented:
netzero.net is not responding to smtp requests -- their mail server may be down.
Pretty standard error, David Johnson nailed it
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Principal Software Engineer
Commented:
"550 Service unavailable" doesn't necessarily mean that the receiving MTA is down.  If the receiving MTA was realio-trulio down, obviously no message would be returned.

"550" is a diffuse message used to mean nearly anything.  Backing reasons range from "I'm busy, come back later" to "We don't want your email but I'm not going to tell you why because that would give away information about our spam filtering."  Much of the time it's the second case - the sending MTA is in a CIDR block for residential cable users, the sending MTA is in a CIDR block used by spammers, the sending MTA has no SPF, the email looks like spam, an external reputation filter doesn't like your domain, and on and on and on.

550 is also used for greylisting, where the receiving MTA wants to see if your email is important enough that you will re-send it.

Without more detailed information the best you can do is shoot in the dark and guess at what it might be.  Have a look at what google returns for "550 service unavailable" and that will give you some idea of how big the pile of potential reasons may be.
Brian BEE Topic Advisor, Independant Technology Professional

Commented:
Basically if you are only having problems with the one recipient domain and nobody else it's probably not your server. It's something they'll have to fix.
David FavorFractional CTO
Distinguished Expert 2018

Commented:
The message "550 Service unavailable" is a generic message which can mean anything.

Might be a problem with the receiving MTA. Might be your sending IP is blacklisted globally or just with the receiving MTA.

To debug this requires.

1) Copy of full email message being sent, with no redactions. Any redactions renders debugging impossible.

2) IP of sending MTA.

3) Mention whether this is the only submission error you're receiving for all users related to the From: domain + IP involved, or this is only a one off problem.

#3 can be phrased better, once you've provided full text + full headers of an email message being sent.

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