A standard envelope is fine, you don't need to use a tax envelope.
Is this answer still correct? do you need to use a tax envelope to mail out a 1099 to someone?
You can send a form 1099 in any envelope as long as you make sure the recipient's federal ID number is not showing outside the envelope, as you are required by tax law to safeguard the recipient's personal identifying information. Many form 1099 preparation software products allow you to only show the last four numbers of a recipient's social security number, and that is a good choice to safeguard their privacy.
Actually, I was a bit surprised to find that the IRS has guidance (rules?) on this. Starts at https://www.irs.gov/pub/irs-pdf/i1099gi.pdf
For example, depending on what's being sent, the statement "important tax return documents enclosed" is required to be on the outside of the envelope.
@Carl wrote:
Actually, I was a bit surprised to find that the IRS has guidance (rules?) on this.
IRS instructions and note the heading:
Statement mailing requirements for Forms 1099-DIV, 1099-INT, 1099-OID, and 1099-PATR, and forms reporting royalties only.