Hello,
I have e-mail service provided by two of the largest telecommunications operators in the world: Deutsche Telekom and La Poste S.A. (France).
There is a strange issue with both systems, where a small proportion of people (maybe 2%?) cannot send me e-mail. So far it has mainly been certain large multinational companies, NGOs, etc.
I have purchased e-mail service from other large operators such as Protonmail, CompuServe and many others. I have never seen this issue before.
To anyone with knowledge of SMTP networking: What would cause a Mail Transfer Agent to fail to send to my ISP MTA?
A spam filter seemed like a simple answer. However, there is no trace of bounced or filtered messages. There is also no pattern as to the blocked organisations or their domain names — and some (multinational corporations) seem far too large to be appearing on spam lists.
Thank you for your assistance.
K.