TLS error for SMTP but not for IMAP service

My domain is:
tuxfriends.net

I ran this command:
certbot renew --force-renewal

It produced this output:
This is hours ago, I forgot it. But there was no error message.

My web server is (include version):
N/A
This is about SMTP and IMAP

The operating system my web server runs on is (include version):
Debian 13.4

My hosting provider, if applicable, is:

I can login to a root shell on my machine (yes or no, or I don't know):
yes

I'm using a control panel to manage my site (no, or provide the name and version of the control panel):
no

The version of my client is (e.g. output of certbot --version or certbot-auto --version if you're using Certbot):
4.0.0

I renewed all certbot certificates today.

And now, my TLS'ed dovecot works fine.

But postfix fails to do the STARTTLS dance.

Thunderbird is showing this error message when I try to send an email (translated from German): unrecognized certificate issuer

Both services are using the same certificate /etc/letsencrypt/live/server.domain.org.pem

In /etc/letsencrypt/live/server.domain.org.pem, I see these renewed certificates:

fullchain36.pem
chain36.pem
cert36.pem

What can I do to debug this mess?

3 posts were merged into an existing topic: TLS error for SMTP but not for IMAP service