A security problem caused by e-mail marketing services
E-mail marketing services such as MailChimp are useful, but people cannot know the difference between a link to a reputable website and a link to a website that installs malware.
"Most viruses, Trojan horses, and worms are activated when you open an attachment or click a link contained in an email message" (www.cisa.gov/news-events/news/virus-basics).
"Some phishing emails are very competently executed to the extent that they are impossible to tell apart from genuine emails just by inspection" (https://webarchive.nationalarchives.gov.uk/ukgwa/20190322230650/https://live.ncsc.gov.uk/blog-post/im-gonna-stop-you-little-phishie).
Many organizations use a marketing service to send e-mails. Usually, each link in the e-mail has one of these structures:
OrganizationName.MarketerURL MarketerURL/OrganizationName
Examples:
http://www.mmsend3.com/spacer.cfm?tracking_id=3759072... https://visitor.constantcontact.com/do?p=un&m=001D... http://r20.rs6.net/tn.jsp?f=001gRwP... https://cornell.us13.list-manage.com/track/click?u=f779b9... http://mailchi.mp/writethedocs/write-the-docs-newsletter-november-2017?e=ef5... https://writethedocs.us6.list-manage.com/track/click?u=94377ea4...
Usually, the e-mails do not have a digital signature. Thus, if you receive an e-mail that seems to come from an organization that you trust, if the links are not to the organization's URL, you cannot be sure that the links are safe.