I have developed a mobile tracking app that uses MailKit and MimeKit to send periodic emails (about the Android phone’s most current location) to an email address ([email protected]). I use an email address created on my dedicated VPS as the sender address.

I discovered that 2 out of the 4 GSM providers in my country (Let’s call them A & B for now) work well with the app as emails are sent out to [email protected] regularly. However, the 3rd GSM provider (let’s call it C for now) appears to have an issue with sending emails to [email protected].

On my VPS, mails originating via my app from A & B appear in the logs and they are always delivered to [email protected] but the mails expected to come from C do not appear on my VPS logs and are not delivered to [email protected].

One of my subscribers on C recently traveled out of the country and roamed his line. He was in Egypt and then China for 2 weeks before returning. All throughout his trip outside the country, his phone via our app was sending messages regularly to [email protected]. He has since returned back and the issue has resurfaced.

Another subscriber recently installed the app in another country using a GSM provider that belongs to the same franchise as C (let’s call it C2 for now) and his phone sent emails to [email protected].

So, where is the issue coming from?