Some emails from the Postfix server return

I run a little greeting cards web-site on VPS. It uses Postfix to send greeting cards to clients. Currently the volume is tiny, about 10-20 cards per day. There is absolutely no spam activity (at least to my knowledge) and my IP is not blacklisted in such organizations as MAPS, www.abuse.net, etc.

I noticed that there are cases when the emails are not delivered. There are 3 major types of faults:

1. User mailbox rejections

Messages:

• 550 Message was not accepted — invalid mailbox
• Recipient address rejected: User unknown in relay recipient table (in reply to RCPT TO command)
• The e-mail address you entered couldn't be found. Please check the recipient's e-mail
  address and try to resend the message. If the problem continues, please contact your
  helpdesk.
• 550-5.1.1 The email account that you tried to reach does not exist. Please try 550-5.1.1 double-checking the recipient's email address for typos or 550-5.1.1 unnecessary spaces. Learn more at 550 5.1.1 http://support.google.com/mail/bin/answer.py?answer=6596
  bk8si48484441wjb.30 (in reply to RCPT TO command)
• etc

I get this for different mail servers including gmail, yahoo, mail.ru, etc. I admit that some users may make a typo but it appears that the rate is way too high, I cannot believe there so many typos can be made.

2. Greylisting

One server reports this:

host mxs.ukr.net[195.214.192.100] said: 451
http://ukr.net/mta/std3.html?my_ip (in reply to RCPT TO command)

The link goes to the page on that server where they suggest "to properly setup queue runners".

3. Unauthorised requests

host mx9.i.ua[82.144.223.46] said: 451 Unauthorised request. Try again later (in reply to RCPT TO command)

Here are my questions, applicable to my case:

1. How difficult would be to fix these issues?
2. Would you recommend to hire a professional IT-specialist to configure email server?

Any help is much appreciated.