If you ever receive  a message from a friend that has nothing but an internet link, that is also a sign that your friend's email has been hacked.  A friend will usually tell you what a url leads to or mention why s/he sent it.
The hackers are active. Last week, I received 4 of those and when I contacted my friends, every one of them had been hacked.

When your email server asks you for a second email address or cell phone number so you can be contacted by another means, be sure to enter one.  When I was hacked, the person changed my email password, so the only way I had to contact my server was through a separate email account.

I changed my password to 16 digits (the longest my server will allow) of random letters, numbers and symbols ----haven't had a problem since. 

Carol Peiffer