External exchange port 25 once all mailboxes are moved..

New Contributor



We have moved all our mailboxes to Office 365.  Our MX records point to Office 365.  Is there any reason to have the external port 25 open on our in house Hybrid server?



6 Replies
Probably not if you’re not running any devices or applications with smtp submission on port 25 ( not recommended ) some legacy devices and apps don’t support recommended port 587 for this and may use port 25!


Just to be clear when you say "running any devices or applications with smtp submission on port 25" you mean external devices or apps, Correct?  


That I know of we are not running any.



I mean for example devices like MF printers sending out mails to people! A common way for this is to use smtp to send, but most devices today is using port 587 for this!
Also if you use any other ways for devices in your environment to send mail please check this! Direct send and smtp relay could utilize port 25 as well!
Except from this I think you’re safe
Please have a look at the link below regarding the ways to setup smtp with devices, and do a checkup!!

an MFP printer would be internal to our network.  I am looking at the old inbound port 25 that email came into.

Best Response confirmed by Bill Lohrer (New Contributor)
Ok, if you’re not using on premises mail servers (hybrid) you’re safe to block it :)

Cheers / adam