solved Email Delivery Log message: Unable to connect to MX servers: []?

After sending out a password reset email to members who migrated from our Ning to our JR site, I checked the email delivery log and saw that some of them failed, with a message that said "Unable to connect to MX servers:"

Can someone tell me what that means? What are MX servers and why wouldn't they connect? does that mean that the servers for recipients are blocking my emails? Or that those people are no longer on those servers (e.g., they may have left their jobs)? I think some of these people are still at those emails, so I'm curious why it's not working.

Any thoughts (in layperson language please).
thanks!
Deb
updated by @icw: 02/20/16 05:58:22PM

An MX server is a Mail Server - this error means when Mailgun tried to contact the mail server for that email address, the MX server did NOT respond - that means it is either down or no longer exists, which in turn means that the email you are sending to may no longer be valid if that server does not come back.

Hope this helps!


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Brian Johnson
Founder and Lead Developer - Jamroom
https://www.jamroom.net

Thanks, Brian... that explanation is very clear and very helpful. Now I can go back and look at the emails that failed with this message and try to figure out what is going on with them.

No problem!


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Brian Johnson
Founder and Lead Developer - Jamroom
https://www.jamroom.net

icw- see this related thread where I have recently been having a lot of these similar email fails: https://www.jamroom.net/the-jamroom-network/forum/my_posts/35425/some-aol-addresses-clogging-email-delivery-failures

Are your and my issues in fact the same? (see my screenshot in that thread )

I contacted Mailgun and this is what they told me:

"The error that you are seeing is ESP throttling, and this occurs whenever the recipient's email service provider (ESP) has received a large number of emails from a specific IP in a short amount of time. Due to this, they will not accept messages for a certain period of time (usually an hour) until they have processed all the emails from the original batch sent.
The error code of "421" indicates that this is a soft, temporary bounce. Whenever we attempt to deliver a message and the recipient server returns a soft bounce, we will retry delivery for up to 8 hours in the following intervals: 10 minutes, 10 minutes, 15 minutes, 30 minutes, 1 hour, 2 hour and 4 hours. Unfortunately this cannot be adjusted and is hard coded in our environment.
I have gone ahead and moved you to a less busy IP, this should reduce the throttling."

That was 2 days ago, and it's happening less now, but still happening- especially with certain AOL member addresses. I'm not so worried about it right now, because I suspect AOL will fix this when their customers complain more, and also I was assured it would not effect my mailgun 'reputation'. FWIW, most of those failed messages do seem to eventually get delivered after two or 3 failed attempts.


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...just another satisfied Jamroom customer.
Migrated from Ning to Jamroom June 2015