invalid characters sending email (crm 2013)

hi there.

when sending an email with Safari or Firefox using MacOSX, some characters become unreadable, the CRM converts them to character '?' (question mark). (this happens AFTER the email is sent)

the characters in question are, for example, these ones: " ã ; à ; é ; ç", etc, which are used in Portuguese Language.

The Language of the CRM is also in Portuguese.

Any ideas? I'm using CRM 2013 on-premises, SP1 w/ UR1.

Thanks in advance,