A few customers have reported not receiving any New User Activation emails, after checking their spam and junk folders.
We use Sendgrid to send New User Activation emails to customers, which is different than our normal System Notification IP addresses.
We found 1 email security service (Proofpoint) who was flagging our Sendgrid IP.
Ask your customer if they are using Proofpoint as their email security service. If so, ask the customer to request Proofpoint to whitelist our Sendgrid IP 126.96.36.199
Then test by adding a new test user to confirm they receive the New User Activation