We got a help desk ticket yesterday and one of our teacher ask help from us that he fail to search all the new Teams she create this school year when she want to send email to this Teams. Noting show up when she searching from the “to” field. 
I create a test team and try it just now. It is true that all the new teams we create this school year are fail to find on “to” field but all the old teams can still search with this way. 
I open a ticket in MS and finally get confirm with them. Well, there is a new policy release in March and all the new Teams will be hidden from outlook by default and we have to use the powershell to unhide it again one by one.
Let’s try it and see how it works. Open the powershell as the administrator.
Type the following command and choose “A”.
Set-ExecutionPolicy RemoteSigned
Login with your Admin credentials with the following command:
$UserCredential = Get-Credential
Type the following command:
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Type the following command:
Import-PSSession $Session -DisableNameChecking 
Run the following command to check your state for your teams and you can find that the setting “hiddenFromAddressListsEnable” is set the true and this is the reason why we fail to search teams.
Notes: Some special character and even space won’t identify from the command so you need to manually delete it if you have.
Get-UnifiedGroup -identity 20190919-Test | Ft Name,Alias,HiddenFrom*
Run the following command again to change the state for your teams.
Set-UnifiedGroup -identity 20190919-Test -HiddenFromExchangeClientsEnabled:$false
And you will see the Teams state has change to “False” now.
And you can the teams again from “to” filed and also find the teams on groups list on the left.
Well, this is the method to solve the problem. I guess the reason why MS change the default policy in March is they want to force every use the message feature from Teams instead to send an email. Email is more like official tool to use but IM is much easier for chat message. So this article is just fit for some user who still prefer using email to announce everything.