When to use professional e-mail address during PhD application When corresponding with administrators and/or academics regarding graduate school/PhD applications, are there any guidelines about which e-mail address to use (personal vs. professional)?

In the non-academic world, it should be fairly obvious that you shouldn't use the address of your current employer to look for a new job, but this doesn't necessarily translate to academic positions.

On the other hand, an address associated with a position with hard time constraints (such as an address at the university you're doing your Bachelor's/Master's at) seems like a no-brainer, because the people at your department probably hope you get a job somewhere else. 

However, what about positions with "soft" time constraints, i.e.: "If we manage to find money for you by date, we might take you on"?-- On one hand, everyone knows that your future is uncertain and so constantly being on the lookout for future opportunities should be understandable if not even supported... but on the other hand, it entails you don't have "faith" that you'll get an extension at your current place... which means you either don't have faith in your own ability, the ability of the department to get funds, or the ability for anyone to get funds (e.g. in times of general cutbacks).

Lastly, irrespective of whatever stigmas (incorrectly) using one's current professional address may be, using an address at a well-respected organization does have an impact in the academic world over compared to using a personal address despite that academic quality should be based only on its own merit.
I am not aware of any guidelines on this, and doubt it will have any impact whatsoever (after all, your current position/university should be clear from your CV). However, there is another reason for using your personal email: access. If you apply for a job somewhere else, there is a good chance you will eventually lose your current professional email adress. And this type of correspondance can be useful to hold on to.