What image/impression does it convey if I use the admin email alias on my own domain? It is not a personal website but a product site and I run the server but I am also the founder of the company so I sometimes need to email customers, investors, staff, etc.
To me, the expectation would be that the person using the admin@domain is the person in charge of the operations of the website in question - a technical job. You might be wearing multiple hats, but you are communicating with potential customers, investors and so on, who do not necessarily know the structure of your company.
That might make them wonder "why is the IT guy messaging me and could I talk to a product manager or someone similar instead?". That is not what you want.
Please note that the emails like support@domain, sales@domain, admin@domain might even be used by entire departments within the company, so this indicates a certain position rather than a person. It would be a bit ridiculous to have a ceo@domain, though - senior management would typically have their names in e-mail addresses. This is because you are not supposed to pick up a conversation right where it stopped the last time with them once there is a staff change. They are not that interchangeable.
To sum it up: use addresses like support@domain or product_name@domain for functional facets of your company/departments, name@domain for pretty much everything else. In startups, it is common to see things like bob@domain, with Bob being the founder and CEO. Because of that, bob@domain actually sounds more senior than admin@domain and carries more weight, if that is what you were worried about!
Usually, the "admin" username is attached with something that is related to overall organization.
In case you are communicating something about the product / system / organization in a generic way, you can use the admin username. Example: While sending the invoice for payment, informing about system update / downtime etc.
However, if you're sending a targeted email, it's better to have a touch of individual / personal presence, and a specific username like yogesch is better. Let's say, you're responding to a specific question / query, or connecting to someone to check the possibility of business extension - go with individual username, as it provides a face they can connect to while reading / responding to those emails.
As others said admin is associated with the technical side of running the website. I would use name/surname where personal connection with customer or investor is required. But I would go with info, sales etc. if you have high volume sales or you require anonymity, or if several people have access to the email.