I have received an e-mail from unknown sender and when I try to reply, I get a message saying that this address doesn't exist
If You receive a letter from unknown address with a non-relevant information and You don't understand why this was sent to You, don't rush to reply. Most probably this letter is SPAM, which can be quite harmful for Your mailbox.
SPAM is a mass mailing to people who did not sign for such e-mails with a purpose of: legal and illegal goods and services advertisement, anti-advertisement, money fraud and personal data acquisition.
In 99% of all cases, spam-messages don't have a reply adress, so Your response will be sent to a non-existing e-mail that spammer entered in the «From» field. This is why You will receive a diagnostic message MAILER-DAEMON@spammailserver.lv или MAILER-DAEMON@yormailserver.com, in which the mail server will alert You about the non-existing account or DNS error.
Technically, any information can be entered in the «From» field. Actual data about the e-mail and sender can be viewed in the e-mail source.
Keep in mind! Be careful while surfing the Web – don't provide Your e-mail on public websites. Use another mailbox, which has a completely different name, for registering on websites or for communications with suspicious users.
If you were unable to find answer for your question, please write to firstname.lastname@example.org