You’re right, sorry. I had checked /var/log/tryton.log but the error was in fact documented in /var/log/daemon.log. I have to look in my gandi config then.
Jan 20 12:38:06 sda-tryton trytond[30739]: convert /tmp/trytond_onwulwkr/sale.sale.odt -> /tmp/trytond_onwulwkr/sale.sale.pdf using filter : writer_pdf_Export
Jan 20 12:38:07 sda-tryton trytond[30739]: 30739 139886547957504 [2021-01-20 12:38:07,523] ERROR trytond.sendmail fail to send email
Jan 20 12:38:07 sda-tryton trytond[30739]: Traceback (most recent call last):
Jan 20 12:38:07 sda-tryton trytond[30739]: File "/root/tryton/venv/lib/python3.7/site-packages/trytond/sendmail.py", line 39, in sendmail
Jan 20 12:38:07 sda-tryton trytond[30739]: senderrs = server.sendmail(from_addr, to_addrs, msg.as_string())
Jan 20 12:38:07 sda-tryton trytond[30739]: File "/usr/lib/python3.7/smtplib.py", line 888, in sendmail
Jan 20 12:38:07 sda-tryton trytond[30739]: raise SMTPDataError(code, resp)
Jan 20 12:38:07 sda-tryton trytond[30739]: smtplib.SMTPDataError: (550, b'5.7.1 Reject for policy reason RULE3_2. See http://postmaster.gandi.net')
Jan 20 12:38:07 sda-tryton trytond[30739]: 30739 139886547957504 [2021-01-20 12:38:07,642] INFO werkzeug 192.168.0.1 - - [20/Jan/2021 12:38:07] "POST /luthierio/ HTTP/1.1" 200 -
UPDATE: It seems that the email with attachment is considered as a SPAM. If the solution may be usefull to other tryton user I’ll document it here.