Hi,

 the solution from  Desk Enterprise worked for me. 

After a bit of testing the ctx.get() funktion ist the way to go.

Thank you for the help.

<field name="email_to">{{ ctx.get('user', '') }}</field>

Hii,

{{user}} Jinja variable doesn't automatically pick up the context value unless you reference it explicitly as ctx['user'] in the template.

Because when Odoo renders email templates, it uses ctx to refer to the rendering context passed from with_context().

so please try below code 
<record id="email_template_remindermail_aktivity" model="mail.template">

    <field name="name">Remindermail aktivity</field>

    <field name="model_id" ref="mail.model_mail_activity"/>

    <field name="subject">Reminder Task due Today</field>

    <field name="email_from">reminder@reminder.com</field>

    <field name="description">Reminder mail for Task due Today</field>

    <field name="email_to">{{ ctx.get('user', '') }}</field>

    <field name="body_html" type="html">

        <!-- Body is a littel bit longer but is working-->

    </field>

</record>

i hope it is usefull

Hi,

The problem is related to how the context variables are passed and rendered in the mail.template. The email_to field in the template expects a valid Jinja expression, and you're passing {{ user }} which is correct syntax only if the context is set up correctly.

In your code, you're using user=f"{email_adress}", but this should be passed as user in the rendering context, not just the environment context.

Try with the following code.

email_address = "test@dummy.com"

template = self.env.ref("reminder_activity_cron.email_template_remindermail_aktivity")

template = template.with_context(

    email_to=email_address,

    displayname=displayname,

    resName=resName,

    link=link

).send_mail(self.id, force_send=True)

Then in your XML template, update email_to field like:

<field name="email_to">{{ email_to }}</field>

Use context keys that match your Jinja variable names. So if you write {{ email_to }} in the template, then pass email_to in context. Always ensure that force_send=True is used if you want the email sent immediately.

Hope it helps