Hi，
What's the type of those email accounts?
How did you set the automatically forwarding rules?
What if directly setting User 1 to forward all emails to User 3?
Generally, to forward emails automatically, we may follow the instructions in the article below:
https://support.office.com/en-us/article/Forward-and-redirect-email-automatically-9f124e4a-749e-4288-a266-2d009686b403?ui=en-US&rs=en-US&ad=US
Please share the rules you set then we can see if the issue can be reproduced in our environment.
Best regards,
Yuki Sun
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