Can a US visa invitation letter be in an email or must it be an original?

I have been accepted to attend a conference in the US, and they will be covering all the costs (transportation, lodging...), but I only received the invitation by email.
Do US visa offices accept a faxed or emailed invitation letter or I should request an original one?
Best Answer
Note that an invitation letter is not technically required, but it can almost certainly help. From the US Embassy website:
An invitation is not required and cannot guarantee visa issuance. In fact, there are no required documents for tourist/business visa applications.
It's more about documenting your reasons for visit, and evidence that you'll leave in a timely manner, etc.
For tourist/business visas, the most commonly applied for visa class, applicants must be able to demonstrate to the satisfaction of the consular officer that their travel plans fall within the parameters of the requested visa class – limited travel for pleasure or business (but not employment) of short durations. Applicants must also be able to demonstrate that they will depart the United States in a timely manner to return to their residence abroad.
As a result, email or letter won't matter, it's not 'official' it's just additional useful documentation. It should contain useful information that can be checked. From Sampleinvitationletter.info:
the following should be in the letter of invitation.
Your letter should include the following information about the person being invited:
Complete name. Date of birth. The person’s address and telephone number. Your relationship to the person being invited. The purpose of the trip. How long the person you are inviting intends to stay in the United States of America. Details on accommodation and living expenses. The date the person you are inviting intends to leave the USA.
Your letter should also include the following information about the person writing the letter:
Complete name. Date of birth. Address and telephone number in the United States. Occupation. Your status in the United States A photocopy of a document proving your status in the United States.
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Can a letter of invitation be emailed?
An email invitation letter is also fine, if it mentions the required information like your name, passport number, duration of stay, etc. Even better, if it mentions further details like, where would you stay, who would bear the expenses during your stay, etc.Does a visa invitation letter need to be notarized?
Notarization is not required neither is an invitation letter but it may help if proper purpose for visit.Does a visa invitation letter need to be signed?
Requirements for Invitation Letter-Visa Knowledge. To facilitate your application, please provide documents that meet the requirements. For invitation letter, electronic signature and electronic stamp are not accepted. Please sign personally and affix company seal (for company invitation letter) on each page.How can I send invitation letter for US visa?
The Invitation Letter can be submitted by the host at the embassy or consulate along with the visa document file. The host must scan it and send it to the visitor, who will later submit it with the other supporting documents on the day of their appointment at the embassy or consulate.U.S. Visa - Letter of Invitation to Visit USA (Sample)
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