Conditional formating where cell value does not contain

Hi and welcome to the forum

maybe try something like this in your rule...
=ISERROR(SEARCH("@testers.com",A1,1))

yes I understand what you want

When I make CF rules that may be a little tricky, I make them IN the worksheet, next to the data, the formula would be the same as if created in the CF window, just easier to do. in each case, you are looking for a TRUE answer to trigger the CF rule.

Based on that, I created this table...

aabbcc...FALSE --->=ISERROR(SEARCH("aa",A1,1))
aaccdd...FALSE --->=ISERROR(SEARCH("aa",B,1))
eeddff...TRUE ---->=ISERROR(SEARCH("aa",C1,1))
ffgghh...TRUE ---->=ISERROR(SEARCH("aa",D1,1))

Did you use that in a new Conditional Formatting rule, using "use formula"?

edit: take a look at the attached

you'r welcome, and thanks for the kind words . If this answered your question, please take a moment to mark the thread as "solved" - it helps keep things neat and tidy lol, and consider adding the reputation to those that helped (see points 2 & 3 below my 1st post to you)