Code:
"how sure can I be that my e-mails are not monitored""
In short, you can't. E-mail is inherently unsafe. It was never designed to be secure as when it was coded, the modern scale of use had never been envisioned
You've got several areas of concern
1) Security of your account.
This is only as good as the password you use, its potential for guessability, and how you store it. No birthdays / dogs names / kids names or similar. No writing passwords on scraps of paper. Make sure the password reset questions cannot be guessed - an idea is to make sure the answers don't relate to the question, e.g. question "mothers maiden name" answer "your last cars registration number"
Also - and this is important - don't use the e-mail password for all your other websites (eg Ebay, Paypay, tesco....) each needs to be distinct
You also have to realise that workers in ISPs / call centres etc are often seriously underpaid and could well be subject to bribery
2) Security of the network
E-mail is sent in clear, unencrypted, through a relay of mail servers. At any one of those servers it can be read by anyone with access in real time. If you make a habit of using wifi access in hotels etc, then its easy to hack you. All I'd need to do would be to turn my Android phone into a mobile hotspot which appeared to be the hotel's network, then all your messages would pass through my phone and be readable (using the correct software). The same can be done with mobile broadband: the G3 transmission signal can be intercepted in much the same way, and the phone spoofed into switching off encryption of the data stream
The only solution is to encrypt all your e-mails end-to-end using PGP or similar. However only a couple of weeks ago a flaw was discovered in the way in which some systems generate public keys, so even that may have problems
3) security of your PC
How sure are you that your computer is not vulnerable? Its a relatively trivial task to send someone a mail containing a keylogger or trojan or worse, or to fool them into visiting a compromised website. Most people have inadequate security software , leaving their systems easily vulnerable
4) personal security
how trusting are you? Do you let other people know your password? Son/daughter/mother/computer repair man.....? You have to keep that password safe
Just remember that the person most likely to spy on you is a jealous friend or close relative. I've been asked several times to put keyloggers on women's machines by husbands. Never the other way around..... I've always refused
So, to reprise, you need
A highly secure password, which can't be guessed or found easily. Preferably at least 16 characters
Password reset questions which cannot be guessed
Encryption of the mails
Computer security which works
A tight lip
And even then you are at the risk of some zero-day flaw being found in your e-mail providers servers