First off, is it a legitimate warning ?
Try malwarebytes to check that.
Otherwise it may be your virus protection programme. What do you use ?
If you are sure you are entering the right e-mail site address then it should be a false warning, but be wary of links sent to you.
Knowing that, and that AVG is a respected AV programme, I can not rule out they are highlighting what is known as a false positive (thinking they found an issue when they've not) but the best feedback is from folk who also have AVG and whether they can say they have the same warning to your e-mail website.
Apologies but I'm not using AVG. Which e-mail service do you use ?