dovecot lda deliver_log_format question

Hey,

i have an question about the deliver_log_format.

My log looks sometimes like:

Oct 1 09:38:01 dd44414 dovecot: lda(user)<20009><9MYzGVmHdV8pTgAA+v7NQw>: msgid=20201001073801.628713AC05DC@myserver.com: saved mail to INBOX

The deliver_log_format settings are still on default. Are somebody able to explain me this part of the log message? <20009><9MYzGVmHdV8pTgAA+v7NQw>:

What want dovecot to say with this?

Mit freundlichen Grüßen

Yours sincerely

Pascal Rudolf