E-mail warning alert, to message receiver.

[REDACTED]

I sent a message last night and was then contacted by the person who'd received my message. They told me that across the top of the message was an alert in a red box saying;

 "Be careful with this message.  It contains content that's generally used to steal personal information."

Also, just beside this warning was a link to Avast but it wasn't in blue so not a proper hyperlink.
I'd noticed that when I put in the person's e-mail address it came up in red; usually they're in black, so possibly significant?

Any ideas as to what is happening please?

TIA,
Thomas

Vista; Avast Free; Thunderbird e-mail.

[Pondus]

did the message contain a attachment ?

can you get a screenshot of the mail and post here?

have you run a quick scan of your computer with avast and Malwarebytes?

[REDACTED]

Hi Pondus,

Yes, the message did have an attachment.  I'll try to get a screen-shot from the person I sent it to, then I can put it up here.

I did an Avast 'quick scan'(?) but nothing was flagged up.

What do you mean by 'Malwarebytes', please?

Thanks,

Thomas

[Secondmineboy]

Malwarebytes Anti Malware: http://www.malwarebytes.org/