This is how to send emails using an additional address (alias) of your mailbox, e.g. info@company.com instead of firstname.lastname@company.com. Requirement: the feature is enabled for your organization.
New Outlook and Outlook on the web
Open Settings (gear) › Mail › Compose and reply.
In the section for sender addresses, turn on the alias addresses you want and click Save.
Create a new email. If the From field is not visible, choose Options › Show From.
Select the alias under From.
Outlook (classic)
Create a new email and, if needed, turn on Options › From.
Choose From › Other Email Address ….
Enter the alias address and confirm with OK.
Note: If the alias isn't offered or the email bounces, the feature isn't enabled yet. Please contact support.