Office 365 end users configure their accounts on Outlook to access their mailboxes. This person is a verified professional. When I enabled it on a backup-alerts mailbox in our most recent client's tenant, their backup alerts started working. Same error.I've only been using outlook.office365.com as the SMTP server. So, here we will talk about how SMTP, IMAP, and POP are different from each other, and what are settings of Office 365 and some popular email clients.The SMTP uses port 25 by default, though it can also use port 587 and 465. Alternatively you can turn off MFA for that account specifically.MFA is not enabled for any of the mailboxes we use for SMTP authentication.Another possibility is basic authentication was disabled for the user/tenant/protocol.When Microsoft rolled out the Security Defaults feature which I think is now on by default for new tenants, it added a new feature to each mailbox under Active Users > User > Mail > Mail Apps > SMTP Authentication.

So, many factors are to be considered while choosing between IMAP and POP3.As IMAP leaves messages on the server; it consumes more storage space as compared to POP3.
While configuring web mail or Office 365 account on an email client like Outlook, users need to be aware of various settings such IMAP, POP, and SMTP with Office 365. Try swapping that out.That's for incoming(IMAP), but I gave it a shot.

Talking about "basic" vs. "modern" authentication.
Added our backup mailbox as an exclusion in that policy, works now. This person is a verified professional. The access policy does not allow token issuance. For more information, see KB 3169958 Important notice for Office 365 email customers who have configured connectors. Such users often prefer to download all these emails and leave no copies behind on the server.

Was surmising that someone disabled "basic" to tighten security.Not talking about SMTP Auth. See the below links how to use Task Scheduler.After some digging around I found that I could go in to the Azure side of our tenant > azure ad > sign ins, add a column for client app and under legacy auth options for that filter choose SMTP. The access policy does not allow token issuance. Create an inbound connector with IP authentication. When viewing one of those failed logins the reason states "Access has been blocked by Conditional Access Policies. But, many people often don’t understand the configuration settings of IMAP, POP, and SMTP email servers. This broke for all of our in-house stuff as well as most if not all of our clients(that have their own office 365 tenants we manage). This person is a verified professional. I'm absolutely convinced this is something in Office 365, I just don't know what.We had the "Block Legacy Authentication" conditional access policy enabled for POP/IMAP/SMTP on all of our mailboxes which was overriding the per-mailbox SMTP Auth setting. This person is a verified professional. The Office 365 Exchange Server can be configured to allow SMTP Relay for specific IP addresses. This person is a verified professional. We have various in-house and client multi-function printers, backup solutions, etc that have SMTP authentication configured in order to do things like scan-to-email, send backup alerts, etc. Added our backup mailbox as an exclusion in that policy, works now. Just migrated some accounts from an on-prem Exchange server to O365. Office 365 is a cloud platform that businesses use for their day-to-day activities and make their environment more collaborative. Nice.

I click test message and the error I receive says "Unable to connect to SMTP server because of invalid credentials or connection settings." So, if your server is offering less storage space, then POP3 is a better choice.IMAP enables accessing messages from multiple devices, which means if you have an iPhone, Android phone, a desktop, or a laptop, with IMAP you can read your emails from any of these devices.Using an IMAP email server, you can achieve all the synchronization that a user needs with an email account, such as reflecting whatever action you performed on one device.Privacy is a significant concern for almost every user. IMAP access is turned on for those individual mailboxes in the cloud Exchange control panel, but it's not working. Usually, you can get these settings from the email provider, but to make it convenient for you, we mentioned the server settings for most popular email providers along with Office 365.Well, it’s hard to pick one among these for sending and receiving emails. While configuring web mail or Office 365 account on an email client like Outlook, users need to be aware of various settings such IMAP, POP, and SMTP with Office 365.But, many people often don’t understand the configuration settings of IMAP, POP, and SMTP email servers. This person is a verified professional. © Copyright 2020 KernelApps Private Limited. Collaborate for free with online versions of Microsoft Word, PowerPoint, Excel, and OneNote. These options are enough to tell you which email server is a better option for you.

Re: Send Mail (SMTP) through Office 365 with MFA @Jeff Harlow someone answered below, not sure why that took so long. We have now been using the service for several years without issue.