When using Gmail as an SMTP server, the From address is wrong
When using Gmail as an SMTP server, you may find that the "From" address doesn't use the address that is specified in the Mail Server configuration. For example, if you specify "firstname.lastname@example.org", you may see emails being sent as "email@example.com".
This problem only occurs when using Gmail as your SMTP server to send emails.
Google will rewrite the "From" field if you do not own the email address being used. Google explains this in their knowledge base.
You'll need to add an alias to the account Confluence is using to send email. This is particularly true if you're using an email account that doesn't exist. Alternatively, you can create a new service account specifically for Confluence to use.