After configuring your SMTP server for Office365 you receive the following error when trying to send an email via the Web Client.
Error sending client email message to "firstname.lastname@example.org" <email@example.com> : Error sending message body - 550 5.7.60 SMTP; Client does not have permissions to send as this sender [HE1PR0601MB2377.eurprd06.prod.outlook.com]
What is happening is your SMTP server expects the FROM email address to match the SMTP server username credentials.
With public email sharing, the default is for Cerberus to use the user's email address as the FROM in the email message. Your server is rejecting that.
You can either fix your SMTP server settings to allow your account to send emails from any FROM address, or you can go to the Public Shares page (Users >Public Shares) and use the "Send public emails from SMTP server account instead of user email address" option. This will ensure the email FROM is always the Sender Name and Sender Email on the SMTP configuration page.