This label is caused by using a web address in the e-mail where the visible URL differs from the actual one.
This situation arises, for example, if I write "Visit our website at www.prospectea.cz" into my e-mail and turn the "word" www.prospectea.cz into a link pointing to http://www.prospectea.cz.
After the e-mail is sent, a visible address marked this way points somewhere else than it looks like (than the e-mail client, Outlook for example, sees it). For the purpose of measuring clicks, we replace all links after sending with links pointing into our system, and from there we redirect the user to the address originally entered in the link. In reality that creates a situation where the visible address www.prospectea.cz points to http://prospectea.onquadna.com/hg654kj6hg6rgfa768jacxzz9uzd8, for example.
The e-mail client (Outlook for example) therefore suspects that you are trying to trick your recipient by showing them a different address than the one it really points to, and marks the e-mail as "suspicious" or "fraudulent".
If this situation arises, it does not matter whether the e-mails are sent by you or by Google.
The solution is to use the link like this: "Visit our website Prospectea", where I mark the word Prospectea or the whole sentence.
In the e-mail client everything is then fine.
Updated červen 8, 2020