Exchange 2010 won't allow IMAP connections over PlainText authentication

Question

I have IMAP enabled for the server, the IMAP4 service running, and IMAP enabled for the account. This is Exchange 2010 server. Emails work fine, but when I try to login over IMAP I get the following error:

NO LOGIN failed.

When I run IMAP Test I get the following:

[PS] C:\Windows\system32>test-imapconnectivity -Connect Plaintext -MailboxCredential:(Get-Credential) | fl

cmdlet Get-Credential at command pipeline position 1
Supply values for the following parameters:
Credential


RunspaceId                  : d70e56b0-0bd2-4ebe-954f-238ef373e43b
LocalSite                   : Default-First-Site-Name
SecureAccess                : False
VirtualDirectoryName        :
Url                         :
UrlType                     : Unknown
Port                        : 143
ConnectionType              : Plaintext
ClientAccessServerShortName : SERVER
LocalSiteShortName          : Default-First-Site-Name
ClientAccessServer          : SERVER.FE.local
Scenario                    : Test IMAP4 Connectivity
ScenarioDescription         : Connect to server using IMAP4 protocol, search for the test message, and delete it along
                              with any messages that are older than 24 hours.
PerformanceCounterName      : ImapConnectivity-Latency
Result                      : Failure
Error                       : IMAP Error: aIFX NO LOGIN failed.

UserName                    : sanctuary
StartTime                   : 09/08/2020 4:39:11 PM
Latency                     : 00:00:00.2731055
EventType                   : Error
LatencyInMillisecondsString :
Identity                    :
IsValid                     : True

Answer 1

What's the login type of the IMAP protocol in your Exchange Server? You could run the command Get-ImapSetting | fl LoginType to check it:

Did you do the imap connectivity test with an admin credential? Based on my test, if we run the above command with an admin credential, we will get an "Error" IMAP Error: aXXX NO LOGIN failed, at the same time, we couldn't perform an IMAP connection for an admin mailbox.

If you failed to pass the imap test with credential of a normal user mailbox, try to run the command Set-ImapSettings -LoginType 3 to switch the login type to SecureLogin, then restart Microsoft Exchange IMAP service and configure an IMAP account for this user. (Here is an article about configure for your reference: Exchange Best Practices: Secure POP and IMAP Access)