How do I link other email accounts as an email contact?

In Office 365 Small Business, it's possible to create a domain-based email account that directs mail to another email account that's not on the same domain. 

For example, you've purchased Office 365 licences for co-workers needing email on the move or SharePoint but there is a need for a mailbox without these advanced features. In this case, our support team can create an email contact (e.g. user@yourcompany.com) and direct your mail to an external mailbox. Email sent to the email contact is received in the external mailbox. You'll need our support teams to set this up so please 
contact us.

If you want to send from the email contact, you must use software like Microsoft Outlook. These are the details you'll need to use:

Incoming mail server: use the username, password and server details of the external mailbox 

Outgoing mail  (SMTP): smtp.office365.com 
Logon details: user@yourcompany.com / password 
Port: 25 
Requires authentication: ticked