This domain can not be used for testing anything!
If your company uses any e-mail address '@test26.com' for internal tests and that generates more that one automated message
then:
- an automated abuse report/complaint will be sent to that companies email provider
- the sender company will be added to public blacklists containing data about unsolicited e-mail senders
That may result in future problems with sending transactional e-mails and deliverablity of your e-mails.
Please think twice before you use this domain for this purpose.