Eu recomendaria usar a notação de sintaxe de URL para a pasta IMAP, conforme explicado no no manual .
Isso deve funcionar para o nome da caixa de correio:
imaps://[email protected]:[email protected]
Do manual:
This has the advantage that multiple IMAP, POP3 or SMTP servers may be specified (which isn't possible using, for example, $imap_user). The username may contain the “@” symbol being used by many mail systems as part of the login name.