E-mail messages collect their time stamps as they travel across the Internet, and have to cater for time zones.
Quite often the time zone on the PC which origianted or receives the e-mail is wrong, even though the displayed time is right.
First the originating PC puts a time stamp on the e-mail, and marks which time zone relative to GMT (Greenwich Mean Time) sometimes called UTC (Universal Time Clock).
For example, where I live is in the UK, so at the moment I am 1 hour ahead of GMT.
When I send an e-mail it goes to the e-mail servers of my e-mail provider which are in America on Eastern Daylight time, 4 hours behind GMT, and this server puts a time stamp on it before sending to the addressee's server which is in Australia, 9 hours ahead of GMT, where another time stamp is put on it.
Finally it arrives on the destination PC, which calculates its sent time and its received time from the time stamps in the e-mail. It converts them to its local time zone.
Is it any surprise that sometimes you receive an e-mail before it was sent?