The first rule for every such question is simple:
If you have to ask, it's probably a scam
While we cannot possibly examine every possible situation from every angle, online scams are simply far too common to trust someone you have known only on the Internet or even what appears to be a real friend or relative, who may have had their email accounts hacked or might be simply being impersonated. So the basic advice is: don't send out your private details, don't pay for anything and don't send any money for any purposes.
Some common examples are:
- love interest who wants to meet you.
- 'supposed' friend in need.
- inheritance of some distant relative.
- person in distress asking you to take care of their financial assets.
- some government/private fine/tax/loan/offer scam
If you know the person who seems to have sent the email asking for money in person, contact them in another way or have them answer a personal question only they know the answer to, before you accept their email as real.
In many countries there are organisations to whom you can report internet fraud or suspected internet fraud. They also offer advice about common scams and how to recognise them.
In the USA: The IC3 (part of the FBI) (further information from the FBI on romance scams)
In Canada: The Canadian Anti-Fraud Centre
In the UK: Action Fraud
The second rule for every such question is also simple:
google this requirement
A legitimate requirement, if exist, would be found on the relevant official sources, including the airline or Embassy web sites. For example, Indian nationals are required to show USD 500/week to enter Thailand.
Many countries also require visa applicants to show means of financial resources. However cash is rarely accepted as such evidence (since one can just borrow it for a day), and depositing this cash in applicant's bank account a day before applying for a visa usually backfires.