t/s expired cert error

[Voytek Eymont]

since a couple of days one of users reported getting expired certificate
error in TB, looking at the log, I can see like:

Mar 02 21:46:24 pop3-login: Info: Disconnected (no auth attempts in 0
secs): user=<>, rip=111.222.333.444, lip=103.106.168.105, TLS: SSL_read
failed: error:14094415:SSL routines:SSL3_READ_BYTES:sslv3 alert
certificate expired: SSL alert number 45, session=<...>

but, looking at server with
https://ssl-tools.net/mailservers/emu.sbt.net.au it says 'valid' as does
certbot tool

what am I missing ?

Expiry Date: 2021-04-27 12:11:32+00:00 (VALID: 55 days)
Certificate Path: /etc/letsencrypt/live/emu.sbt.net.au/fullchain.pem
Private Key Path: /etc/letsencrypt/live/emu.sbt.net.au/privkey.pem


# grep ssl_  dovecot.conf

ssl_cert = </etc/letsencrypt/live/emu.sbt.net.au/fullchain.pem
ssl_key = </etc/letsencrypt/live/emu.sbt.net.au/privkey.pem
ssl_dh=</etc/dovecot/dh.pem