Marketing through e-mails is pretty effective, yet difficult to execute. One thing about newsletters is that people want what they ordered. People, who for example signed up for news and current affairs, seem allergic to having the content diluted by advertising.
A survey by Statista
found out that 26.8 percent of all American respondents thought ads in newsletters were very annoying. Another 48.5 percent of respondents thought that those ads were either 'not too' or 'kind of annoying'. So, counting together all of the annoyed you're talking about 3 in 4 people who could do without ads in e-mail newsletters. Only 12.2 percent of respondents weren't annoyed at all.