Below a number of characteristics of questionable publishers and journals are listed. Separately, they may serve as warning flags and together they form a basis for a decision not to submit your manuscript to them.
The most distinguishing characteristic of these publishers is, as has been mentioned earlier, that they send email invites that seem very personally directed toward the individual researcher which is a way assure the researcher that the manuscript will likely be accepted ("What? Do you want me to publish in your journal?" That's flattering!"). In the email, a title from an earlier conference paper the author has written might be mentioned. The publishers usually harvest these titles together with the email addresses and paste them into a standardized email message. As these email messages are usually like spam, that is, they send out a lot of theses "personal" invites, they hope that at least some will be lured to publish in the journal.
Example of an email from a questionable publisher:
In the next section, we will discuss a couple of details you could look further into and some resources you can use to assess if the publisher or journal is legitimate or questionable.
Cobey, K. (2017). Illegitimate journals scam even senior scientists. Nature, 549(7670), 7. DOI: 10.1038/549007a
Vence, T. (2017, July 17). Identifying Predatory Publishers: How to tell reputable journals from shady ones. The Scientist. Retrieved from http://www.the-scientist.com/?articles.view/articleNo/49862/title/Identifying-Predatory-Publishers/