According to a member of staff the two incidents took place last week.
The Royal Mail confirmed that post was stolen from a locked, secured van in Borehamwood, but it was later recovered.
Another incident occurred in London Colney, where mail addressed to 12 residential properties was taken from a member of staff while it was being delivered.
A Royal Mail spokesperson said their colleagues were not harmed and the police have been informed.
They added: “We always inform the police of such incidents. The security of our customers’ mail is a high priority for Royal Mail, and incidents like this are quite rare.
“Swift action by the police usually ensures that the missing items are quickly retrieved.
“When it is not been possible to retrieve a stolen item, we aim to contact the customer concerned the next day.”
Customers have been advised to contact the Royal Mail on 03457 740 740 if they believe the incidents have affected them.