Currently, new users are prohibited from using third-party clients by default.
If you want to use a third-party client, you can enable or disable the third-party client login feature.
Meanwhile, we suggest that you may enable the third-party client security password to better protect the account security.
Email is an open product. The openness means that the global mailbox service supports a common standard protocol to access its own services by default. Its manifestation is that users can use any three-party client such as Outlook and Foxmail to access any mailbox. However, the standard protocol only supports the "account + password" authentication mode.
If the enterprise requires employees to login to the mailbox only by using a third-party client in the company, please operate [Disable the third-party client access] and add the company network IP as the secure login IP.
If the enterprise does not allow employees to logon to the mailbox by using a third-party client, perform the following operations: [Disable the third-party clients access] or [Disable the SMTP Service].
Procedure
[Disable Allow Third-party Clients]
The lighted button
means that it is allowed to access third-party clients, and the 
means that it is not allowed to use third-party clients.
1. Log on to the Alibaba Mail with the postmaster account, enter domain management page and choose Security> Account Security Strategy.
2. If you turn off the Allow third-party clients, the members cannot logon to the mailbox through the third-party clients. After the IP address is added in the Company IP Addresses/IP Range, the logons from the added secure IP address are not restricted.
3. Click Save in the upper-left corner to complete the settings.
[Disable the SMTP Service]
1. Log on to the Alibaba Mail with the postmaster account, enter domain management page and choose DEPTs & Users> Batch Settings.
2. Add the Operation Object.
3. In the Batch Settings section, select POP3/SMTP Service Switch and unselect Enable POP3/SMTP and Enable IMAP/SMTP.
4. Click OK.
