use warning level for email and mailbox same domain error. Update email wording.

email_handler.py

@@ -564,7 +564,7 @@ def forward_email_to_mailbox(
 
     # sanity check: make sure mailbox is not actually an alias
     if get_email_domain_part(alias.email) == get_email_domain_part(mailbox.email):
-        LOG.exception(
+        LOG.warning(
             "Mailbox has the same domain as alias. %s -> %s -> %s",
             contact,
             alias,
@@ -575,7 +575,7 @@ def forward_email_to_mailbox(
             user,
             ALERT_MAILBOX_IS_ALIAS,
             user.email,
-            f"Your SimpleLogin mailbox {mailbox.email} cannot be an email alias",
+            f"Your mailbox {mailbox.email} and alias {alias.email} use the same domain",
             render(
                 "transactional/mailbox-invalid.txt",
                 name=user.name or "",

templates/emails/transactional/mailbox-invalid.html

@@ -1,19 +1,27 @@
 {% extends "base.html" %}
 
 {% block content %}
-  {{ render_text("Hi " + name) }}
-
   {% call text() %}
-    We have detected that your mailbox <b>{{ mailbox.email }}</b> cannot receive emails as it's a SimpleLogin alias.
+    <h1>
+      Your mailbox {{ mailbox.email }} and alias {{alias.email}} <b>cannot</b> have the same domain.
+    </h1>
   {% endcall %}
 
   {% call text() %}
-    A mailbox needs to be a "final" email address and cannot be an email alias.
+    The alias domain should point to SimpleLogin servers so emails sent to it are forwarded by SimpleLogin.
   {% endcall %}
 
   {% call text() %}
-    Please update this mailbox to a non-alias email address on
-    <a href="{{ mailbox_url }}">{{ mailbox.email }} setting</a>.
+    The mailbox domain should be your "final" email address that can receive emails from SimpleLogin
+    and cannot point to SimpleLogin.
+  {% endcall %}
+
+  {% call text() %}
+    Please either: <br>
+    <ul>
+    <li>update the mailbox to a final email address on {{ mailbox_url }}</li>
+    <li>remove {{alias.email}} domain from your custom domains</li>
+    </ul>
   {% endcall %}
 
   {% call text() %}

templates/emails/transactional/mailbox-invalid.txt

@@ -1,11 +1,15 @@
 Hi {{name}}
 
-We have detected that your mailbox {{ mailbox.email }} cannot receive emails as it's a SimpleLogin alias.
+Your mailbox {{ mailbox.email }} and alias {{alias.email}} cannot use the same domain.
 
-A mailbox needs to be a "final" email address and cannot be an email alias.
+The alias domain should point to SimpleLogin servers so emails sent to it are forwarded by SimpleLogin.
 
-Please update this mailbox to a non-alias email address on
-{{ mailbox_url }}
+The mailbox domain should be your "final" email address that can receive emails from SimpleLogin
+    and cannot point to SimpleLogin.
+
+Please either
+- update the mailbox to a final email address on {{ mailbox_url }}
+- remove {{alias.email}} domain from your custom domains
 
 Feel free reply to this email if you have any question.