Best practices for sending messages and not being marked as a spammer:
To avoid being labeled as a spammer when sending bulk mail, define nrcpt in:
This lets you control the number of recipients per SMTP session depending on destination domain.
Create an SPF record
SPF records help define who can and cannot send mail from your domain.
Use DomainKeys (DKIM) to further legitimize your messages
DKIM is a signing method that use DNS to access to public keys. Because Sympa modify the message before to brodcast them, it may alter existing DKIM signature. http://en.wikipedia.org/wiki/DKIM
More things you can do to prevent being added to spammer blacklists: (see the spam filters section)
MailChimp: Designing, Coding & Delivering HTML Email