Attn Walrus

[matxer]

I wouldn't want to meddle in your private discussion, but I think the problem is just that the connection between provider A and provider B is closed. But not with other providers. So provider A can send / receive emails from provider C, D, E... Same with provider B.

The solution is that just one of you use an alternate email.

It could even be a more local problem. If a hacker forges the email address of Walrus and sends spam to the provider of Vesku, then the latter blocks the connection, so that Vesku can't receive / send email to Walrus.

Whatever it is, I suspect it is due to clumsy anti-spams rules from ISPs.