Exchange

Outlook
Fixing Autocomplete for To,CC, and BCC fields if not or stops working

In Outlook client navigate to File > Options > Mail (scroll down)

Under Send Messages check "Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines". If already checked, uncheck click OK then go back in and re check the option. Verify it is working.

SMTP
Error:

There is no valid SMTP Transport Layer Security (TLS) certificate for the FQDN of "server". The existing certificate for that FQDN has expired. The continued use of that FQDN will cause mail flow problems. A new certificate that contains the FQDN of "server" should be installed on this server as soon as possible. You can create a new certificate by using the New-ExchangeCertificate task.

Fix: 

Open Exchange Management Shell and type New-ExchangeCertificate > yes