Tag: 4xx

Asynchronous Bounces

There are three ways that an email can fail to be delivered: immediate rejection timeout asynchronous bounce Rejection A rejection is any delivery attempt where the sending smarthost can tell immediately that the mail can’t be delivered. That will often be when the receiving machine accepts a connection but returns a “hard bounce” or “5xx” […]


Thoughts on bounce handling

This week’s Wednesday question comes from D. What are your thoughts on bounce handling There’s no specific standard for bounce handling as we often talk about it. While a lot of people will mention the RFCs, the RFCs only say what to do with a single email that gets a 4xy or a 5xy during […]

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Changes at Gmail

As I’ve said before, I can usually tell when some ISP changes their filtering algorithm because I start getting tons and tons of calls about delivery problems at that ISP. This past month it’s been Gmail. There have been two symptoms I’ve been hearing about. One is an increase in bulk folder delivery for mail […]


AOL transmitting 4xx error for user unknown

AOL is currently returning “451 4.3.0 <>: Temporary lookup failure” in some cases when they really mean “550 user unknown.” This message from AOL should be treated as 5xx failure and the message should not be retried (if at all possible) and the failure should be counted as a hard bounce for list management purposes. […]

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Soft bounces and rate limiting

What is your policy for handling soft bounces? What do you consider a soft bounce? What is the right thing to do about soft bounces? The first step in talking about soft bounces is to define them. When I talk about soft bounces, I mean mail that has been rejected with a 4xx response during […]

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