You may know you have multiple options for who is the sender on your Pardot emails. They are as follows, from the least to the most restrictive:
General User (any email address you want)
Specific User (a specific named Pardot/Salesforce user)
Assigned User (the person that owns the Salesforce lead or contact)
Account Owner (the person that owns the Salesforce account)
You also have the ability to use a cascading fallback strategy for who the sender is, because of the nature of Salesforce objects and which can be explained as follows:
The most restrictive is Account owner, because the Pardot prospect could be a lead (no account) or not in Salesforce at all. It's also possible that the person is an orphaned contact (no Account).
The second most restrictive is Assigned User, because the prospect might not be in Salesforce, and therefore not assigned, or they could be a lead in a queue in Salesforce and not assigned yet.
In both the cases above, Pardot will insist that you use a fallback Specific user or General User in case there's no assignment.
The Inactive User Risk
A user is made inactive in Salesforce most commonly when they leave the company. Their license is returned to the pool, and their user profile says "this user isn't active" but their user account remains in Salesforce, and unless the administrator finds a new owner for their records-- they remain in the ownership of the inactive user. Herein lies the problem.
So what happens in Pardot when a user is made inactive in Salesforce? Absolutely nothing. The Pardot records retain the same ownership they had in Pardot before, it's just that now the Salesforce user isn't active. So if you send an email that references the owner, Pardot still sends the message as if that person who left your company is still there. Because Pardot sends the email by proxy it still is sent as if that inactive user sent the message.
...Not the best idea if your star salesperson who went to a competitor is still sending sales emails as if they worked for you...
The same thing can happen if you use the account owner as the sender-- an inactive user will send the message. This can even happen if the sender is a named user if that user is inactive as well.
So not only does your prospect see the sender as someone who doesn't work for you anymore, if they reply to the email they may get a hard bounce, or worse, if you have an "accept all" server, your mail system might just "eat" the incoming message, send no indication it wasn't delivered, and the message goes to....nobody. Now do you see why this is so important!?
Another hidden area where issues could also crop up is even if you are sending from a general user but are using any of the account owner or assigned user merge tags in the message to say "your sales contact is".
What to do about it?
The most important thing as marketers that we can do is to make sure we know who the sender is on all active email templates in Pardot. Do an audit and check. Especially if you are working with a Pardot instance that you inherited. The nice thing about Pardot is the assets give you a "where used" at the bottom of the page, so you can see what email templates are used where.
The second thing is if you use, or intend to use, Assigned User or Account Owner as a sender or in merge fields, do an audit in Salesforce for unassigned records. You may need your admin to help you with this one, especially if you work in a large org with many users-- it's sometimes hard to keep track of the comings and goings of staff.
What you want to do is run a lead report and an account & contact report where you can use the "active" checkbox referring to the record owner (either the lead or the contact or the account) as a filter. This field may or may not be available on any report you can create-- therefore your administrator may have to give you access to this field and custom reports that will show it.
Essentially what we want is the field that is available by lookup to the account owner, contact owner, and lead owner via the user object that is called "Active". Be careful-- your org may well have a standard field on contacts or accounts called "Active"- that's not it! These screenshots show how an admin would add these fields to a custom report type of "Accounts with or Without Contacts" that incorporates the Contacts > Contact Owner > Active field and the Accounts > Account Owner > Active field.
Therefore, what you get if you run a report of this type filtering for if the owner is not active (false), are all the Accounts, Contacts or Leads owned by a user that is not active. You'll probably need to run a report for just the contacts and just the accounts separately.
If your admin is on the top of their game, ideally all these reports would come up blank, zero, empty, no records found. In other words, no records owned by inactive users. If you do find records with inactive owners, then you need to work with your admin and your sales team to decide who should the records belong to, so you're not sending Pardot email from someone who left the company six months ago.
One Final Caution
You thought you were done? One more thing you should check: Active users that you don't want sending email. Are any of your records owned by the Pardot Integration user (B2BMA)? This is also an issue because that user doesn't have a workable email address.
Do you have any records "parked" with users that would be inappropriate as email senders? For instance, admins, IT support, external vendors, the CEO, etc. You need to be aware of any of these "don't send from" users too. Create a salesforce report that looks for any of these users too.
How can I keep on top of this?
A simple thing to do is build yourself a quick dashboard using the metric (record count) component-- one for each report for your "no no" senders and simply set yourself reminder to check it once a week. If all goes well, everything should remain at 0 (once you refresh the dashboard). If you start seeing numbers, it's time to dig into record updates with your admin.