SMTP & API

How to Fix Error 5.7.1 (Relay Access Denied)

Code 5.7.1 « Relay access denied » means the SMTP server you connected to refuses to forward your message to...

How to Fix Error 530 (Authentication Required)

SMTP error 530 means: I won’t accept your message until you log in. The server is asking for authentication and...

How to Fix Error 552 (Storage Limit Exceeded)

SMTP error 552 means the destination has refused the message because it exceeded a storage or size limit. Two distinct...

How to Fix Error 451 (Local Policy Violation or Greylisting)

SMTP error 451 is a soft bounce: the receiving server is asking to try again later. The two most common...

How to Fix Error 553 (Mailbox Name Not Allowed)

SMTP error 553 means the receiving server has rejected the mailbox name in the envelope. Either the address is malformed...

How to Fix Error 4xx (Temporary Failure)

An SMTP 4xx code is a temporary failure: the receiving server is saying « not now, try again later ». The address...

What Are the Most Common SMTP Errors?

SMTP errors fall into two families: 4xx codes are temporary (retry), 5xx codes are permanent (don’t retry until you fix...

How to Solve Error 550 (Requested Action Not Taken – Mailbox Unavailable)?

SMTP code 550 is a hard bounce: the receiving server has refused the message. The text right after the code...

How to Solve a DNS Code 5.7.26 Error?

Error 5.7.26 is a Gmail rejection that means: this message did not pass authentication. Since Google’s 2024 sender requirements, any...

What Is a Monitoring DMARC Policy?

A monitoring DMARC policy is the safest way to start with DMARC. You publish a TXT record with p=none, which...

How to Make Sure Password Reset Emails Don’t Go to Spam

To stop password reset emails from landing in spam, you need to combine three things: technical authentication, content discipline and...

What Is a Password Reset Email?

A password reset email is the automated message a service sends when a user clicks « forgot password ». It...

What Is the Difference Between SMTP, IMAP and POP3?

SMTP, IMAP and POP3 are the three protocols that move email around the internet, and each has a single, distinct...

Can SMTP Receive Email?

Strictly speaking, SMTP receives email at the server level — it’s how mail servers accept incoming messages from other servers....

What Is SMTP Error 554 and How to Fix It?

SMTP error 554 is a permanent rejection: the recipient server refused to accept your email. The good news? Once you...

Is SMTP protocol reliable?

Yes — SMTP is one of the most reliable application protocols on the internet. It runs over TCP, supports retries,...

Which SMTP port to use?

SMTP uses several ports, but only two of them really matter for sending email today: 587 (with STARTTLS) and 465...

How Does the Mailpro API Work?

Mailpro’s API lets developers send transactional and marketing emails directly from their software, without going through the web interface. Access...

How Do I Send Emails with an SMTP Server?

Sending email through Mailpro’s SMTP server takes about ten minutes from start to first message. The exact steps depend on...

How to Manage Unsubscribe Link for SMTP Messages?

Even when you send through SMTP from your CRM or your own application, every commercial message in 2026 must include...