Can't Find altemail In Saved Search Filter
Can't Find altemail In Saved Search Filter — a practical guide from Script Everything.
Why can’t you search for altemail in a Saved Search filter?
Whether you’re coding using SuiteScript or just using the Saved Search feature in Netsuite you might find it difficult to use the altemail field in Netsuite, as it doesn’t exist as a search feature in the Filter section of Netsuite’s documentation.

So how do you search for this email address for your Individual Customer records where this field is used?
Simply use the email filter and it will both search the altemail field and the email filter.