If existing mail delivery platform (e.g. Exchange, Office 365, Gmail) is performing any reputational/connection level/spamchecks on inbound SMTP servers ensure Email Security addresses are whitelisted to prevent incorrect spam identification or potential delays when Email Security relays mail onwards.
Note
When using Email Security in a GSuite environment careful consideration has to be taken with regards to internal G Suite messages.
For more information on this topic see the below links: