Don't worry too much. There are lot of ways to either get your mail address directly and there are also a lot of ways so that your address can get stuck on some spam mail list. This is nothing special. Use a strong password and everything will (probably) be fine.
Anders' answer is perfectly fine, because it shows what a hacker can do with one address. I just want to give some example of how emails can be found and spread today. When you see, how easy it is for perpetrators to get to an email, you will probably discard your worries.
There are two scenarios. Either you want a big ol' list of addresses, let's say 100,000 to send out spam, advertisement or w/ever. Or you want one specific e-mail, let's say yours.
Scenario 1: Big ol' list
Here are some ways to get a loooot of valid mail addresses.
- Compromise accounts.
Just try googling "list of valid mail address". You will find tons of lists, with 10 to 100,000 entries. Most of them probably won't be valid, but I'd say half of them are. I don't for what kind of ratio someone might be looking for who sends out spam, but I guess that wouldn't be ideal.
So let's google for something else, how about "ashley madison email list". The first site you hit, gives a step-by-step instruction on how to download the complete list of 30+ million mail addresses. A lot of them are fake, deleted and won't work anymore. But just change "ashley madison" in the search query with any service name that a major breacht in the last 2 years. I guess most of the mails will still be active there.
Don't trust other people's lists? Make your own!
You start with two lists. The first lists has the 100/500/1000 most common first names and the second one - you guessed it - the 100/500/1000 most common last names for the country of your choosing. Combine each entry in both lists with another in this way and add the domain of the 5-10 biggest mail providers at the end. Now you will have a list with (100/500/1000)^2*5 entries of "[email protected]". I guess 95% of these will fail but you did it yourself!
With all the mails that failed, you can then start to add random numbers at the end and stuff like that, but that probably goes too far.
Each mail account has an address book or a contact list. A lot of mail accounts are compromised everyday. You probably heard of the big breach at yahoo, where 3 billion(!) accounts were compromised. If only a third of these accounts had 10 contacts in their address book (probably with more than just an mail address. We're talking full name, date of birth, address.) and you would be somehow able to compile a list of all these contacts in all these contacts list, you would have a list with 10 billion addresses. There would obviously be a lot of duplicates, but 10 contacts for a third of the accounts is a pretty small number.
So you see, compiling a big list with a lot of valid addresses is rather easy. Your and my mail address is probably in such a list right now.
So what if someone wants to get your specific private mail address?
There are several ways for attackers to get to your mail address. This can include social engineering, hacking mail accounts of your contacts (to get to their contact info) and also a lot of guesswork. Sometimes web services display addresses of users, so if I know a service like this, I can try to look you up. Also googling is a pretty good way most of the times if an attacker knows either your real name or an online handle you regularly use. A lot of people publish their address on sites of sports clubs or fan sites. These are all pretty well known ways, there are more, but if I reeeeaaaaally (for whatever reason) want to get your address and you have used it sometime in the past, I could probably get it somehow.
The question is though: is somebody out there, who whants your email this badly? I guess, and I hope for your sake, that this is not the case.
Is this a privacy issue?
First of all, if YOU don't want anybody to get your email address, don't give it to people. People share all kinds of stuff unasked that touches your privacy: phone numbers, photos, videos etc. Additionally most people - especially in a professional environment - don't see sharing of contact information as a privacy violation. I personally - and I guess most people on this SE - have several mail addresses. One for work, one for web services of some kind and one for another.. you get the point. If you want to keep something private, don't share it with people.
Everything changes, when you live under a suppressive regime or have some other reason, to really worry about your privacy. Then especially look into "is this a privacy issue".