Which of the following is the most common method for an attacker to spoof email?

A back door is a covert access mechanism installed on a compromised system and has no direct role in forging email sender addresses.

A replay attack captures and retransmits legitimate network packets but does not involve forging email headers or using SMTP infrastructure.

A man-in-the-middle attack intercepts live communications between two parties but is not the standard technique used to forge and send spoofed bulk email.

An open relay is an SMTP server that is misconfigured to forward email from any source without requiring authentication, allowing an attacker to set an arbitrary sender address and relay messages globally. This is the most common email spoofing method because open relays are relatively easy to find and abuse, effectively hiding the true origin of the message. Attackers chain multiple open relays together to further obscure the source.