What's the difference between congruent and incongruent email interruptions?

Despite the negative connotation associated with email interruptions in the popular press, not all email interruptions are necessarily bad. It is essential to distinguish between congruent and incongruent interruptions.

Interruptions that are congruent with one’s core work responsibilities can ultimately have positive effects on individual performance. However, because they still interrupt the current task, congruent interruptions also increase people’s mental and emotional workloads, which is detrimental to performance. 

So, congruent interruptions can improve performance but at the cost of increased stress.

On the other hand, incongruent interruptions (e.g., emails concerning secondary work activities or personal issues) have negative effects, because they both hinder work performance, and increase people’s mental and emotional workloads (which further hampers work performance).

See more background to the research that underpins this at the MIS Quarterly website