During my day to day work as a part of support organization, I work with and help troubleshoot mailbox migrations very often. One type of migrations that we see quite often is IMAP migrations. I want...
Updated Nov 21, 2023
Version 5.0
Blog Post
A good post but I don't see my error listed 
I'm trying to migrate some users from a "mail.uk2.net" IMAP service to Exchange Online. All users have Microsoft Apps for Business licenses with the exception of myself. As the global admin. I have the Teams Preview license and also I have added two Outlook migration roles, discussed in the docs.
In preparation, I have added the MX and Txt records to Office 365 for the source host. Then I tried the migration wizard but this just errors with a whole list of possible causes then don't make sense.
Thought I would have a go at the PowerShell
Test-MigrationServerAvailability -Imap -RemoteServer mail.uk2.net -Port 993 -Security Ssl
<#
RunspaceId : d9718fa4-147e-49b6-a3f5-bf26de82e24e
Result : Success
Message :
SupportsCutover : False
ErrorDetail :
TestedEndpoint : mail.uk2.net
IsValid : True
Identity :
ObjectState : New
New-MigrationEndpoint -IMAP -Name IMAPEndpoint -RemoteServer mail.uk2.net -Port 993 -Security Ssl
New-MigrationBatch -Name IMAPBatch1 -SourceEndpoint IMAPEndpoint -CSVData ([System.IO.File]::ReadAllBytes("c:\dev\Clients\Myclient\admin\migratedusersonly.csv")) -AutoStart
Domain controller LNXP265A001DC06.GBRP265A001.PROD.OUTLOOK.COM is overloaded. Please try again later.Anybody seen this "lovely" error?
Update: the error is actually temporary tho can reoccur at any time. I have 'progressed' to the next error. Both the user's password and the SSL connection seem fine.
MailboxEmailAddress : firsname.surname@domain.co.uk
RecipientType : Mailbox
SkippedItemCount : 0
SyncedItemCount : 0
MailboxGuid : 6e8b8fae-b69d-4857-a455-?????????????
MailboxLegacyDN : /o=ExchangeLabs/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn??????-firstname.lastname
RequestGuid : 00000000-0000-0000-0000-000000000000
LastSuccessfulSyncTime :
Status : Failed
StatusSummary : Failed
MigrationType : IMAP
State : Failed
Flags : None
WorkflowStep : DataMigration
WorkflowStage : Injection
TriggeredAction : None
DataConsistencyScore :
HasUnapprovedSkippedItems :
ErrorSummary : The username or password for this account is incorrect, or IMAP access is disabled. --> The username or password for this account is incorrect, or IMAP
access is disabled. --> Imap server sent NO response to AuthenticateCommand. Response code: '', message
IsValid : True
ObjectState : Unchanged