You shouldn’t see that - it’s in the translation files because %{…} is a replacement key. The double %% overrides that, e.g.:
errors:
invalid_integer_min_max: "Value must be between %{min} and %{max}."
…
site_settings:
reply_by_email_address: "Template for reply by email incoming email address, for example: %%
{reply_key}@reply.example.com or replies+%%{reply_key}@example.com"
Where are you seeing this error? Also, what version are you on?
On our hosted sites we override these values as they’re specific to our hosting. It’s possible this check is outdated, we’re looking into it.
This is your SMTP server saying “your authenticated user doesn’t own this address, you can’t use it.” You’ll want to check with the administrators of your SMTP server to determine what can be changed there to permit this.