When searching or grouping a list of leads based on email address filter ‘Contains’ @ sign, you may encountered the different numbers in the ‘People’ tab (result).
This behaviour is specific to the email filter as it uses two main indexes; we’ll call them Index 1 and Index 2 when using the contains/not contains operator.
Index 1 searches for the entire email address value from beginning to end. This Index is used when the @ character is not included in the list of values.
Index 2 searches only for the domain half of the address, which comes after the @ symbol. This Index is used when the @ character is included in the list of values.
A suggestion is to create a smart list with two Email Address filters wherein you will have all the ones that do not have the '@' symbol, and for the other, you will include the '@' symbol for the email address and use the logic as 'Any'. That would give you the preferred number of leads in your smart list.
Marketo database searches for email domains in a way that when using the 'contains' keyword in the 'Email Address' filter, you must make sure that if you are using the '@' symbol in your list of domains that all email domains contain this. If you want to filter for email domains that do not include the '@' symbol, you must make sure that no email domains in your list use the '@' symbol.
This behaviour happens when the indexing of the database and the rules that apply to the contains operator.