Can an email field be constrained or filtered to only allow specific email address domains to be submitted? For example, the survey would allow @gmail.com and @yahoo.com but error on any other values. Is that a thing you can implement in the logic of the .xlsx file from Connect?
Yes, this is possible using Survey123 Connect and configuring a constraint on the email field. You can also use regular expressions to ensure the format of the email is correct, so combining these will ensure the format is correct and only allowed domains are entered.
In the documentation there is example of how to set up email address regex: Formulas—ArcGIS Survey123 | Documentation
And if you search GeoNet you will see there are plenty of posts and a few blogs regarding this topic.