HOTSPOT
A company plans to deploy an Office 365 tenant.
You have the following requirements:
Administrators must be able to access the Office 365 admin
center.
Microsoft Exchange Online must be used as a Simple Mail
Transfer Protocol (SMTP) relay for a line-of-business application that
sends email messages to remote domains.
All users must be able to use the audio and video capabilities in
Microsoft Lync 2013.
You need to configure the ports for the firewall.
Which port should you use for each application? Select the correct answer from each list in the answer
area.
Answer: 
correct
SMTP relay is 587
Port 587 (recommended) or port 25
https://support.office.com/en-us/article/How-to-set-up-a-multifunction-device-or-application-to-send-email-using-Office-365-69f58e99-c550-4274-ad18-c805d654b4c4?ui=en-US&rs=en-US&ad=US&fromAR=1
I agree Port 587
port 25 is only second option
Port 25 is the correct answer. From the link above from Luigi, scroll down and check the SMTP Relay section “Option 3: Configure a connector to send mail using Office 365 SMTP relay” and not the SMTP section.
Which port SHOULD you use ? the answer is in the details. Should = recommended. 587
Port 25 is correct for SMTP Relay (Option 3 from the Article https://support.office.com/en-us/article/How-to-set-up-a-multifunction-device-or-application-to-send-email-using-Office-365-69f58e99-c550-4274-ad18-c805d654b4c4?ui=en-US&rs=en-US&ad=US&fromAR=1)
Port 587
https://support.office.com/en-us/article/Office-365-URLs-and-IP-address-ranges-8548a211-3fe7-47cb-abb1-355ea5aa88a2?ui=en-US&rs=en-US&ad=US#bkmk_exo
Purpose: Required: client SMTP Relay
Port: TCP 587
According to the article that Luigi provided, the port required for an “O365 SMTP relay” is port 25.