Anything other than the accepted format is not imported, this is one way of rooting out invalid email addresses.
Enterprise accounts are set up in a pool of IP addresses, which enhances delivery of emails. Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and domain keys are set up on these dedicated domains, and are authentication tools that work to legitimise email sending. For example, SPF matches the IP address you're sending from to the domain you're sending from, and confirms that you've given the system permission to send emails on your behalf. Your subscribers's receiving servers will then accept the mail as authentic.