As your email ends in .de I gather that's the German keyboard thing/conflict with @ symbol and marking low priority.
You can modify the respective /opt/zimbra/mailboxd/webapps/zimbra/WEB-INF/classes/keys ZmKeys_de.properties & ZhKeys_de.properties files to contain:
compose.LowPriority = Alt+L
compose.LowPriority.mac = Ctrl+L
Then: zmmailboxdctl restart & clear your browser cache.
Bug 28221 - shurtcut "Alt+L" coflicts with "@" char on german mac keyboard