Date: 2024apr24 OS: Linux Q. Postfix: warning: TLS library problem: error:0A000416:SSL routines::ssl/tls alert certificate unknown:ssl/record/rec_layer_s3.c:865:SSL alert number 46: Q. For me, this exact error happened during STARTTLS when my server (Postfix) had a self-signed certificate so the client rejected it. You can fix it by: - Getting a proper certificate for your mail server (free from Lets Encrypt) Or - Making the client accept self-signed certificates.