I recently did a newsletter and had proofread it three times and had an associate check it as well. Somehow, we both missed a typo. The email went out to about 1000 clients, friends, and some family. While a lot of people didn’t mention it, a couple people did. ( I am sure others noticed it but were too polite to mention it)
In this case, as the error was in the link text of a URL, it wasn’t picked up by my spellcheck.
So for instance https://microsof.com wouldn’t be picked up as an error even though it is missing a “t” and Microsoft would be usually picked up in a dictionary.
My mistake was that although I read through it, the link in question was only one letter off and it got by me (and of course the spell check).
While this wasn’t a major mistake, to prospective clients, finding typos does not portray you in a good light.
When you are working on print or emails, remember once they are out there, they are out there forever. So, any mistakes you make cannot be fixed. Take the extra time needed and make sure there are no mistakes, either grammatically or even within your hyperlinks.