What email address goes in "From Address" field for email notifications?

Don't!

Make sure that the emails themselves contain a "from" address that will lead back to the relevant server and script in the event that a malfunction (such as rapidly-repeating emails) causes mail problems. Failure to sort out flooding issues may result in loss or closure of your email system and could be costly for your business. I usually use something like
<scriptname>.<servername>.<plant>.<country>@<company-name>.<tld>

It will also make it look like someone competent and organised is running the show!

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