I configured now and restarted dekkod as:
phablet@ubuntu-phablet:~$ cat ~/.config/dekko2.dekkoproject/dekkod.conf
[FileLog]
Enabled=1
Path=/home/phablet/dekkod.log
[LogCategories]
IMAP=0
Messaging=0
POP=0
SMTP=1
[StdStreamLog]
Enabled=0
[Syslog]
Enabled=0
phablet@ubuntu-phablet:~$ restart dekkod
dekkod start/running, process 26955
The resulting log is very good and I'm familiar with such SMTP logs from sendmail and other MUA:
[24252] Feb 02 12:39:16 [Debug] Running action TransmitMessagesRequest 107185203838979
[24252] Feb 02 12:39:16 [Debug] SMTP : newConnection
[24252] Feb 02 12:39:16 [Debug] SMTP : Open SMTP connection
[24252] Feb 02 12:39:16 [Debug] SMTP : RECV: "220 ms-10.1blu.de ESMTP Exim 4.86_2 Ubuntu Sat, 02 Feb 2019 12:39:15 +0100"
[24252] Feb 02 12:39:16 [Debug] SMTP : SEND: "EHLO [127.0.0.1]"
[24252] Feb 02 12:39:16 [Debug] SMTP : RECV: "250-ms-10.1blu.de Hello ppp-88-217-102-34.dynamic.mnet-online.de [88.217.102.34]"
[24252] Feb 02 12:39:16 [Debug] SMTP : RECV: "250-SIZE 104857600"
...
Now I have just to wait and catch the next send resulting in Drafts ...