If you rarely use e-mail, then after a while you can completely forget it. Although this does not happen often, there are proven ways to find out your email address.
Try to use the content of the site that provides you with mail. Let's say you have not logged out of your account and your login is available to you. Most often, it is the main component of the email address. Add a dog sign and a domain to it. For example, if the login is vopros, and the site is kakprosto.ru, then the e-mail will be [email protected]. To find out if you were mistaken in your guesses, try sending some letter to this mail.
Some sites provide a choice of several domains to login. If you cannot remember it, you can try all the options. As a rule, there are no more than four of them. Follow the same rules - send a letter to yourself and choose the one that will reach the recipient.
If you left someone your email address, then contact him and ask him to remind your e-mail. You can also ask your friends if they remember your email. Often, people close to you know your address and can help. It is also worth contacting colleagues at work who can send you important information.
If you have access to mail, but you do not remember your own address, just look in the Sent Items folder. As a rule, in the field "from whom" your e-mail will be indicated, you can also contact the technical support service of the site, which in 99% of cases will help to resolve the situation.