Issue SSL/TLS for E-Mail does not show up

[stesan]

Hi,

in Plesk Obsidian 18.0.29 / psa v18.0.29_build1800200731.19 os_CentOS 7 - everything is good - that is what i'm expecting ( plesk-email-certificate01.PNG ) OK.
in Plesk Obsidian 18.0.28.3 / Debian 8.11 in Plesk - the LetsEncrypt Dropdown-Box for E-Mail SSL/TLS does not show up. ( plesk-email-certificate.PNG ) NOT OK.

Does anybody know why the dropdown box does not show up and how to fix this.
Thanks for help.