Best practice suggestions for email to customers:

Begin the subject with ZEISS.  This will help the customer clearly see it is a valid email (and very important!!!)

Include a serial number where applicable

Include a machine description or information that will help the customer find your email in a search later

It's all about making this easier for our customer.  Otherwise, there is a higher probability that they will misplace your email.  Similar best practices apply for internal correspondence.  Make it easier for the receiver (and for yourself to find the sent emai later).