NEW BRUNSWICK, N.J. — As anyone with an e-mail inbox knows, the "system administrator" message is the return-to-sender stamp of the electronic age. Its subject line, "undeliverable," announces a fatal error in sending an e-mail — perhaps a sloppy keystroke, a misheard address or a cancelled account.

At the trial of nurse Melanie McGuire Thursday, prosecutors offered one such bounced-back e-mail as proof of something much more grave: Murder.

The e-mail at issue, described to jurors by a state investigator, was sent from a Blackberry belonging to McGuire's husband, William, the morning after authorities allege she shot him to death.

"I will be out sick today," the April 29, 2004 message read.

The writer attempted to send the message to two of William McGuire's supervisors at the New Jersey Institute of Technology, where he worked as a computer analyst. Only one of the men received the e-mail, however. The address of the second was incorrect and resulted almost immediately in an "undeliverable" notification from the system administrator, state Division of Criminal Justice official Donald Macciocca testified.

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He said the sender typed the only the first name of the supervisor, Thomas Terry, into the address when, in fact, Terry used his last name.

Prosecutors have portrayed the wrong address as strong proof that it was Melanie McGuire, not her husband, who wrote the e-mail. According to his co-workers, William McGuire was very familiar with the e-mail accounts because he oversaw the e-mail server and determined his colleagues' addresses.

On cross-examination of the investigator, a defense lawyer suggested that whatever the victim's expertise, William McGuire may simply have entered the address incorrectly in his Blackberry. Macciocca acknowledged that when investigators recovered the Blackberry from the victim's trunk, Terry's e-mail was wrong in the device's address book.

Lawyer Steven Turano also noted that Terry's address was different from those of other members of the department, who used their first names as addresses, and therefore may have been difficult to remember.

McGuire, 34, is accused of killing her husband, dismembering his body and dumping it in the Chesapeake Bay. She was having an affair with her boss, a fertility clinic physician. She insists she is innocent, and her defense has suggested the victim, a frequent visitor to Atlantic City casinos, was murdered over alleged gambling debts. If convicted, she faces a possible life sentence.

Melanie McGuire listened Thursday to testimony from her former lover, Dr. Bradley Miller.

On Thursday, Macciocca testified about an assortment of phone, e-mail and other records he reviewed during the investigation. He said that after April 28, 2004, there were only two outgoing calls from the victim's cellphone. One was to a Navy buddy on April 30, and the second was to the family home on May 2. Both calls were listed by the phone company as a minute in duration. The prosecution has implied that Melanie McGuire made the calls after killing her husband to establish a defense.

Macciocca said the May 2 call occurred about 15 minutes after E-ZPass records show Melanie McGuire's car passing through a toll near Atlantic City.

Two months after her husband's disappearance, McGuire admitted to her lover that she had gone to Atlantic City in April and May to look for William McGuire and as a prank, moved his car and took his cellphones, which she later threw in the garbage.

According to records displayed by the investigator, at about the same time as she told her lover about her trips, she tried to get toll charges for two trips through the Atlantic City tolls removed from her E-ZPass bill.

"Customer stated she did not travel the ACE on two different dates," Macciocca read from an E-ZPass record.

A customer service representative refused, the investigator said.

Also Thursday, McGuire's former lover, Bradley Miller, completed his testimony in the case. Under questioning from a defense attorney, Miller described McGuire as beloved by patients and admired by colleagues.

"Was she a caring individual in general?" attorney Joe Tacopina said.

"Yes," he said.

"That is one of the things, obviously, that attracted you to her?" Tacopina said.

Miller nodded.

Testimony resumes Monday morning. The trial is being shown live on Court TV Extra.