Failed to send message
Error: impossible to establish a secure link with the outgoing server (SMTP) box.mydomain.com using STARTTLS because it doesn’t announce that mechanism. Deactivate STARTTLS for this server or contact your service provider.
I started having this issue in the afternoon after basically two years without problems with MiaB and Thunderbird. The problem seems to be related to network connection (ISP). When I use my home’s network, Thunderbird shows me that error when trying to send emails, but I can receive mails normally.
As soon as I share my phone’s data to my computer, the error disappears and the email is sent. Email can also be sent with my phone’s mail application using mobile data plan. I also connected my computer to a VPN and the email works without problems.
I then tried to check for fail to ban logs to see if the IP provided by the ISP for my home was blacklisted, but it was not on the MiaB logs. I also accessed the Web interface without problems and sent an email.
Finally I though that my ISP blocked SMTP outgoing port 587, but I used linux telnet and nmap utility and found that the port is not blocked.
Can somebody please tell me other diagnostic that I can try to solve this problem?