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The Public IM connectivity setting controls instant messaging (IM), audio, and video communication with contacts who use the Windows Live public IM service provider. Public IM connectivity is disabled by default.
To use IM in this new E1 plan with another account on the P1 plan, you need to enable domain federation for the new E1 account first, and then enable Public IM for it. To do so, you can refer the following steps:
Step 1: To configure federation with external domains
1. Log on Office 365 Online Portal, click Admin page. Then click Mange under Lync Online. In the Lync Online Control Panel, click External communications.
2. Under Other organizations that use Lync, review the current setting for Domain federation mode.
3. To change the current setting, click Edit, choose from one of the following options, and then click OK:
a. Turn on external communications with all except blocked domains
b. Turn off external communications with all except allowed domains
c. Turn off domain federation
4. To add an allowed or blocked domain, click Add domain, type the domain name, and then click OK.
The status of the domain—allowed or blocked—is determined by the current Domain federation mode setting.
5. To remove a domain from the list, check the box next to the domain name, click Remove domain, and then click Yes.
Step 2: To configure Public IM connectivity
1. In the Lync Online Control Panel, click External communications.
2. Under Public IM services providers, note the current value of Public IM connectivity mode.
3. Click Enable or Disable as appropriate, and then click Yes.
Note: The organization you’re federating with must also set up federation with your domain.
For the detailed information, you can refer the following articles:
Configure federation with external domains
Configure Public IM Connectivity
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