Is it sinful for spouses to send sexually explicit messages to each other via smartphone and email? My wife did send a sexually explicit picture to me but we have deleted all traces of it.

As to the particular issue, I’m afraid it is naïve to believe that “all traces” of such material can be deleted from electronic devices. Once created and sent, such data lasts forever. As a rule of thumb, you should never send through email or text anything you would be uncomfortable to find plastered on a freeway billboard. Even before the technological advances of the cyber age, it was difficult to destroy all traces of such material once created.

As to whether or not this is moral, the morality of the matter depends on the content. If it was of a third party, then it constitutes the use of pornography. Even if it was of one of the spouses, the context of your question suggests that impure activity may have accompanied the exchange. Either way, this is something that should be confessed.