554 Email rejected SMTP error

How to diagnose 554 smtp errors and configure your mail client to send email correctly to custom domains with MailSlurp

  • Table of contents

When sending or receiving emails in MailSlurp you must understand that there are two types of inbox:

  • SMTP Inbox -- Typically has address ending in @mailslurp.mx or a custom domain you have created -- Can be accessed via SMTP and IMAP (see your dashboard for username and password)

  • HTTP inbox -- Usually ends in @mailslurp.com or a custom domain that you use. -- Still has SMTP server but cannot be accessed directly with username and password -- No IMAP support as uses AWS SES behind the scenes

Diagnosing your inbox error

The first step to find out the source of a 554 email error is to determine that your inbox is type SMTP:


Find the MX records for an address

If your inbox ends in @mailslurp.mx we can use the dig command to find the MX records associated with it.

dig -t mx mailslurp.mx

; <<>> DiG 9.16.1-Ubuntu <<>> -t mx mailslurp.mx
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 52885
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

; EDNS: version: 0, flags:; udp: 65494
;mailslurp.mx.			IN	MX

mailslurp.mx.		600	IN	MX	10 mx.mailslurp.com.

;; Query time: 76 msec
;; WHEN: Thu Jan 20 21:31:08 NZDT 2022
;; MSG SIZE  rcvd: 73

Notice the answer section contains an MX record pointing to mx.mailslurp.com. We can telnet to the server to test SMTP access.

Telnet to SMTP server

MailSlurp SMTP server runs on ports:

  • 25
  • 2525
  • 456
  • 587

Use telnet to connect to MailSlurp:

telnet mx.mailslurp.com 2525

The result of which looks something like this:

Connected to mx.mailslurp.com.
Escape character is '^]'.
220 mx.mailslurp.com SMTP Server (MailSlurp) ready

If you see this the server is running and can be reached. Next we will try send an email to an inbox.

Test email receiving

First create an SMTP inbox using your API_KEY and SDK or the dashboard.

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