==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.6.2
- Deployed with iRedMail Easy or the downloadable installer? Downloadable installer
- Linux/BSD distribution name and version: Ubuntu 20.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? No
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

Hello!

Mine it's a fresh installation & inbound emails work just fine.

When I try to send I get "SMTP Error (454): Authentication failed."

I then receive an email with:

Transcript of session follows.

Out: 220 mail.DOMAIN.TLD ESMTP Postfix
In:  EHLO localhost
Out: 250-mail.DOMAIN.TLD
Out: 250-PIPELINING
Out: 250-SIZE 15728640
Out: 250-ETRN
Out: 250-STARTTLS
Out: 250-ENHANCEDSTATUSCODES
Out: 250-8BITMIME
Out: 250-DSN
Out: 250-SMTPUTF8
Out: 250 CHUNKING
In:  STARTTLS
Out: 454 4.7.0 TLS not available due to local problem
In:  QUIT
Out: 221 2.0.0 Bye

For other details, see the local mail logfile

Log is as follows.

Dec 29 18:34:30 mail postfix/submission/smtpd[19396]: warning: cannot get RSA certificate from file "/etc/letsencrypt/live?/mail.DOMAIN.TLD/cert.pem": disabling TLS support
Dec 29 18:34:30 mail postfix/submission/smtpd[19396]: warning: TLS library problem: error:02001002:system library:fopen:No such file or directory:../crypto/bio/bss_file.c:288:fopen('/etc/letsencrypt/live?/mail.DOMAIN.TLD/cert.pem','r'):
Dec 29 18:34:30 mail postfix/submission/smtpd[19396]: warning: TLS library problem: error:20074002:BIO routines:file_ctrl:system lib:../crypto/bio/bss_file.c:290:
Dec 29 18:34:30 mail postfix/submission/smtpd[19396]: warning: TLS library problem: error:140DC002:SSL routines:use_certificate_chain_file:system lib:../ssl/ssl_rsa.c:615:
Dec 29 18:34:30 mail postfix/submission/smtpd[19396]: connect from localhost[127.0.0.1]
Dec 29 18:34:30 mail roundcube: <hoedltrp> PHP Error: Invalid response code received from server (POST /mail/?_task=mail&_unlock=loading1672000871750&_framed=1&_action=send)
Dec 29 18:34:30 mail postfix/cleanup[19398]: 4NjcVt6gSSz3ygg: message-id=<4NjcVt6gSSz3ygg@mail.DOMAIN.TLD>
Dec 29 18:34:30 mail postfix/qmgr[19242]: 4NjcVt6gSSz3ygg: from=<double-bounce@mail.DOMAIN.TLD>, size=832, nrcpt=1 (queue active)
Dec 29 18:34:30 mail postfix/submission/smtpd[19396]: disconnect from localhost[127.0.0.1] ehlo=1 starttls=0/1 quit=1 commands=2/3
Dec 29 18:34:30 mail roundcube: <hoedltrp> SMTP Error: Authentication failure: Invalid response code received from server (Code: 454) in /opt/www/roundcubemail-1.5.3/program/lib/Roundcube/rcube.php on line 1778 (POST /mail/?_task=mail&_unlock=loading1672000871750&_framed=1&_action=send)
Dec 29 18:34:30 mail postfix/cleanup[19398]: 4NjcVt72RCz3ygh: message-id=<4NjcVt6gSSz3ygg@mail.DOMAIN.TLD>
Dec 29 18:34:30 mail postfix/qmgr[19242]: 4NjcVt72RCz3ygh: from=<double-bounce@mail.DOMAIN.TLD>, size=974, nrcpt=1 (queue active)
Dec 29 18:34:30 mail postfix/local[19404]: 4NjcVt6gSSz3ygg: to=<postmaster@mail.DOMAIN.TLD>, orig_to=<postmaster>, relay=local, delay=0.06, delays=0.03/0.02/0/0.01, dsn=2.0.0, status=sent (forwarded as 4NjcVt72RCz3ygh)
Dec 29 18:34:31 mail postfix/qmgr[19242]: 4NjcVt6gSSz3ygg: removed
Dec 29 18:34:31 mail postfix/pipe[19405]: 4NjcVt72RCz3ygh: to=<postmaster@DOMAIN.TLD>, orig_to=<postmaster>, relay=dovecot, delay=0.1, delays=0/0.01/0/0.09, dsn=2.0.0, status=sent (delivered via dovecot service)
Dec 29 18:34:31 mail postfix/qmgr[19242]: 4NjcVt72RCz3ygh: removed

I don't see the "?" ("/etc/letsencrypt/live?/mail.DOMAIN.TLD/cert.pem" - log: line 1) in /etc/postfix/main.cf.