E-Mail Activity date in red - what does this mean?

Hi everyone,

I’ve got a question about the coloring of a field. We’ve sent out e-mails with invoices to customers. The e-mail show up as processed in “All Emails” (CO409070) with no errors. So far so good.

Now we have received some complaints that invoices have not been received. For each complaint  we’ve checked the activities in the invoice (AR301000) and here the date is shown in red. 

I couldn’t find any information on what this means. Any help or pointing me in the right direction would be much appreciated!

Thanks.