I've been asked by bankers, school administrators, insurance agents, car salesmen, realtors, you name it to send them all kinds of information over e-mail over the years. Everything from my address and phone number to credit card information and Social Security Number, either as text or in the form of scanned documents. Depending on the situation, sometimes I've said no, and sometimes I've said yes.
I know regular e-mail is insecure in theory while in transit, but not much more than that. A lot of times companies/people seem concerned only about convenience, so they make it hard to find other ways to send the information, like regular mail or faxing. So I'm wondering, is the danger in sending this information via e-mail much greater than the danger of those other ways?
I did a bit of homework before asking, and here's my current (but potentially flawed) understanding. There are two categories of vulnerability here, network sniffers at either endpoint and less-than-trustworthy servers on the route between the endpoints. From what I've read, it seems like the greater danger in practice is sniffers at endpoints (e.g. realtor checking mail on a public coffee shop network), although this is mitigated if people at both ends are on relatively safe (i.e. private) networks and/or using HTTPS-enabled webmail.
As for the other threat, I have a vague understanding of networking, and I wouldn't know where to begin getting control of machines that actually route e-mails between their start and end points. The danger there feels lower to me both because it seems more technically challenging and because such a machine would probably process a high volume of mail, so, "security" through obscurity might apply.
The alternatives are also hard to judge. Regular mail seems secure (if slow) but unless it goes to the final recipient, it might get scanned and insecurely e-mailed anyways. Faxing seems about as tangly as e-mailing, just over a different network, but probably harder for an attacker to "tap" in a useful way?
So, in addition to my main question above, I'd appreciate if you could correct any misconceptions I have there.