Sending email failure

||Operating system|CentOS Linux 7.9.2009|
|Webmin version|2.013|
|Usermin version|1.861|
|Virtualmin version|7.5 Pro|

Hi All,
I have encountered a strange email issue as follows.

Users can receive email with no issues.
When sending email they gat an error. At the time I didn’t write the error down but from memory that was error 10064 (can’t connect to server ?).

The users have recently moved home so a new internet connection. They have 4 devices that worked ok with their old home but none can send from their new connection.

Connecting using another, for example using their mobile phone as a hot spot (and thus away from their wifi and using a different ISP) they can send with no issue.

Because of that, I assumed that the problem was with the ISP but lengthly talking to them (excellent support at Zen) we could find no error on their systems. The only thing I could find that was changed for the user was the fact that their new connection had IPv6 enabled so in desparation got that disabled at their router.

That solved the problem, email could now be sent without any issue.

The domain they were sending via does have a v6 ip munber.

I have a suspicion that the same is happening for at least one other user.

Any ideas would be very welcome.

Thanks for reading.

Does your server have a IP4 address?

Thanks Stefan,
Yes, it does have IPv4.

Another symptom that I omitted to mention is that email can be sent if the user disables their anti virus. But AV can be left on when the router has IPv6 disabled.

what anti virus software is being used … from this that appears to be the culprit here … may some research in that direction may help you

Thanks for your reply Jim,

I did search for a solution about AV (AVG and Avast are the two AV programs that I know are used) but I didn’t get mutch traction.

However I do remember that some time ago (a long time ago) there was a problem sending emails to Gmail addresses with an error similar to “the sender is not adheering to basic IPv6 rules” or something like that. Searching produced a solution to change “inet_protocols = all” to “inet_protocols = ipv4” in postfix/ That did reolve the issue of emails being sent to Gmail.

I’m guessing that this is what is causing my problem now.

Has anybody got any guidance/suggestions on this?

Thanks for your time

I’m not so sure IP6 is the problem.
Is it only Gmail accounts that are affected? I’d try to rule that one in/out first. If it is just Gmail I would have though there would be a lot more noise on the net about it.

What email client are they using? (have they also moved to new “computer” as well as a new home, is it a new service provider, is it just a new router/building)

Thanks for your input all.

I think the problem is now resolved. In case this comes up for someone else here are the symptoms and events in chronological order.

Some time ago I was getting errors when the server tried to forward emails to Gmail accounts. From memory the error was something like “the sender is not adheering to basic IPv6 rules”. I searched for a way to resolve that and found a few suggestions that I should change “inet_protocols = all” to “inet_protocols = ipv4” in postfix/ That did solve the problem.

Fast forward to the present and some users were having difficulty sending email to any address (error 10064). An example is a user that moved home taking four devices that all worked without issue at their old home but failed when using their new connection at their new home. Also worked when using their cell phone as a local hot spot. No changes had been made to their email settings. Same service provider just a new connection. I assumed a problem with their provider.

At their new home if either IPv6 or anti virus was disabled or they used a different connection sending email was no problem.

I now remember the Gmail issue and the change that disabled IPv6 in postfix/ from some time ago and wondered if that was connected to the current problem.

This morning I returned postfix/ to the original “inet_protocols = all” and tested that email could be sent to Gmail. And it can. I’m guessing that either Gmail have changed the restriction or something has been updated on my server.

So I have enabled the users router to enable IPv6 and so far the problem seems to have gone away.

I don’t understand the reason why anti virus should affect IPv6 but anyway, all appears to be working now.

Thanks for reading this long post.

I guess that something has timed out or similar because now I am back to the same issue that the user cannot send email unless the IPv6 is disabled at their router. Or AV disabled.

That doesn’t appear to be causing any other issues (please let me now if you think something will crop up later) so I guess they will have to leave it as it is.