The email address is valid. I have tested it against the regular expression stored in the host setting "Security_EmailValidation" which is:

\b[a-zA-Z0-9._%-+']+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}\b

I am now unable to update my host settings because of this error. It used to work but I suspect a couple of updates I did today:

1) Upgrade  Forms & List module to version 6.01.00

2) Upgrade Feedback module to version 6.04.03

I have not changed the host settings for some time, but the last time I did it, there was no error. Today, after upgrading these two modules, I could now update the settings.

My dnn version: 7.4.1