When using SBO mailer to send e-mails from SAP, there is some special considerations regarding using Office 365.
SAP Business One uses the e-mail from the username as the sender address. Due to the current solution, the SMTP account to actually send the e-mail is only one for the entire application. This often causes the sender e-mail and the SMTP e-mail not to match.
In Office 365, this is prohibited by default and the e-mails will get stuck / never be sent.
In order to correct that, there's a configuration that has to be done in the Office 365 server. It will basically say that one e-mail account has the permissions to send as another one.
Section copied from Microsoft knowledge-base:
Send email on behalf of another user
In the admin center, go to the Users > Active users page.
Select the name of the user (from whom you plan to give a Send on behalf permission) to open their properties pane.
On the Mail tab, select Manage mailbox permissions.
Next to Send on behalf, select Edit.
Select Add permissions, then choose the name of the user or users that you want to allow to send email on behalf of this mailbox.
Here's the reference Microsoft article: