There are two ways to separate domains and usernames stored in one column in a spreadsheet.
Example: Column B stores firstname.lastname@example.org and the goal is for column C to store only domain.com.
This method provides one column with just the username and another with just the domain. In the example, column B would contain only "username", overwriting the complete address, and column C would contain only "domain.com".
Select the column containing the email addresses (column B in the example) > go to the DATA tab in Excel's toolbar > click "Text to Columns" in Data Tools in the toolbar > set Type to "Delimited", click next > set Delimiters to Other and enter @ in the box (uncheck any other Delimiters that are checked), click next > set Column data format to "General" and click Finish.
The formula below can be added to another column, replacing "B" in the example with whichever column contains the email addresses and updating rows accordingly.
This leaves the source column intact, outputting only the "domain.com" portion of an email address. Note that 255 is the number of characters.