Hi Robert,
I can send you more information here to speed up:
in "Events", for example, on the rule "User Disk Space Used"> Action "Send an email and notify the user" the text that is written (with line break or other html elements) in the form as follows:
The mailbox at #emailaddress# is #mailboxusagepercent#% full.
Remove unnecessary emails or attachments to prevent messages from being blocked.
(Space Used: #mailboxusagemb# MB out of #mailboxsizemax# MB)

in
notifications of the
webmail is shown:
<div>The mailbox at #emailaddress# is #mailboxusagepercent#% full. </div>
<div>
<br>
</div>
<div>Remove unnecessary emails or attachments to prevent messages from being blocked. </div>
<div>
<br>
</div>
<div>(Space Used: #mailboxusagemb# MB out of #mailboxsizemax# MB)</div>
That is, the html is not decoded within the "notifications" view of the web mail.