SPF (Sender Policy Framework)

links: wiki syntax tools validator RFC 4408

DNS records

where:
ip4:ipAddress - Allowed IP for send

DKIM (DomainKeys Identified Mail)

links: wiki validator RFC 4871

generate DKIM keys

configure exim

configure exim for internet site

edit config: /etc/exim4/update-exim4.conf.conf
or (if u use splitted config) /etc/exim4/conf.d/transport/30_exim4-config_remote_smtp

add this line:

restart exim

DNS records

mail._domainkey IN TXT v=DKIM1; k=rsa; p=publicKey

where:
p=publicKey - u public key without new line breaks and comments (/etc/exim4/dkim/domain.tld.pub)
mail._domainkey - DKIM_SELECTOR from exim config