OK, just sort of a toungue in cheek reminder here...
Everyone at your company using email for your domain name to do business needs to have an email signature set up. Practically every email program or service out there lets you do this. It's so that people can contact you easily. If you don't have it, you're making it more difficult for them to contact you. It's a basic thing but I'm surprised how many people I come into contact with are not doing this.
Make sure it lists your website as well so that you get more traffic to your website.
An email signature should contain:
To make sure your emails show up looking correctly (you want clear communication), go with plain text.
My biggest pet peeve is email programs that add in line breaks. They get so difficult to read after a while - turn them off and you'll get people responding to you more quickly.
Absolutely not. Your logo is good to have there for branding but if you're sending it as an attachment every time, then you're doing it wrong. There's a better way to do it so that it's not seen as an attachment each time. Sometimes I'm looking at emails for an attachment someone is sent me and I have to go through all of their emails because they ALL have attachments... which just happens to be their logo or social media icons.
Having a proper email signature set up is basic. This should almost go without saying but it's good to make sure it's set up. You'll see more business going to your website and your customers and vendors more happy to communicate with you. Even setting up a company-wide standard is a great idea so that email signatures are consistent across your company. Everyone sends out and replies to a LOT of email, so make sure you're doing it right.