Você não pode (confiavelmente). Você pode escapar do ponto (usando um \) no nome da caixa de correio, mas isso nem sempre é uma opção. Fugir do ponto não é oficialmente padronizado em qualquer lugar que conheço e, embora seja amplamente suportado atualmente, não há qualquer garantia de que ele será apropriadamente analisado por qualquer consumidor dessa informação.
Veja os seguintes recursos:
link email-addr
Email address of the person responsible for this zone and to which email may be sent to report errors or problems. In the jargon this is called the RNAME field which is why we called it email-addr. The email address of a suitable DNS admin but more commonly the technical contact for the domain. By convention (in RFC 2142) it is suggested that the reserved mailbox hostmaster be used for this purpose but any sensible and stable email address will work. NOTE: Format is mailbox-name.domain.com, for example, hostmaster.example.com (uses a dot not the more normal @ sign, since @ has other uses in the zone file) but mail is sent to [email protected]. Most commonly ending with a '.' (dot) but if the email address lies within this domain you can just use hostmaster (see also example below). when to use the dot.
In DNS (see [RFC1033], [RFC1034] and [RFC1035]), the Start Of
Authority record (SOA RR) has a field for specifying the mailbox name of the zone's administrator.This field must be a simple word without metacharacters (such as "%" or "!" or "::"), and a mail alias should be used on the relevant mail exchanger hosts to direct zone administration mail to the appropriate mailbox.
For simplicity and regularity, it is strongly recommended that the well known mailbox name HOSTMASTER always be used
.