If you are trying to adjust an email account that Plesk tells you already exists, but you can't find in the list of email addresses for the domain, it's likely that the address is configured as an alias. To check this, do the following:
- Login to Plesk via the Client Centre
- Choose Mail in the sidebar on the left and, if prompted, select the appropriate domain
- At the top of the list of mail addresses (mailnames), select "Show Aliases"
You should now see all aliases as 'sub' (indented) addresses under their parent address - this is the address they are an alias of.