Yahoo! seems to have security issues with their email service. There may also be other factors.
• Change your account password ASAP. Use more then ten characters. I have one 'pacbell.net' account with twenty-three characters.
• A memorable password can be created using numeric substitutions, a mix of letter cases, and some special symbols. "#5tR1ng%0shUn&" is memorable for me ("string", "ocean", and symbols), and just as strong as "fsaGljh0KLhn".
• Don't use the same password on multiple accounts; even with other services.
• Re-evaluate your security questions. I generally am dishonest about my answers. "Twirler" is a better answer than "Dodge" for the question, "What was the make of your first car?" The honest answers to most security questions can be guessed through judicious Internet research on your name. Granted, you will need a way to track your lies; but it isn't illegal to give a phony answer when the bank asks for your mother's maiden name.
I include this last because some people (not necessarily you) get suckered:
• Your provider will never ask you to send your login details by email, through a web link, or a cold call. Never comply with such requests.
Some of the "other factors" include friends who email to address lists without using Bcc:. If an email with such a list is in the message store of a computer infected with a Trojan, or the like, those email addresses will be harvested, and reported back to the "Mother Ship". Do use, and try to get your correspondents to use, "Bcc:" for multiple recipients.
You might want to reconsider keeping your contact list online.
I once had an email address forged by a spammer. It caused problems for a couple of weeks, until I worked out a strategy to deal with the backscatter
If you are up to it, buy a personal domain (typically about $6 to $10 a year), and email hosting (sometimes included with the domain). You get to keep your email address when you change providers, and you usually have better control over spam filtering.
If I think of anything else, I will post updates.--
~Oh Lord, why have you come
~To Konnyu, with the Lion and the Drum