The Reply-To field inherits reply-to StructuredField and handles the Reply-To: header field in the email.
Sending reply_to to a mail message will instantiate a Mail::Field object that has a ReplyToField as its field type. This includes all Mail::CommonAddress module instance metods.
Only one Reply-To field can appear in a header, though it can have multiple addresses and groups of addresses.
mail = Mail.new mail.reply_to = 'Mikel Lindsaar <mikel@test.lindsaar.net>, ada@test.lindsaar.net' mail.reply_to #=> ['mikel@test.lindsaar.net', 'ada@test.lindsaar.net'] mail[:reply_to] #=> '#<Mail::Field:0x180e5e8 @field=#<Mail::ReplyToField:0x180e1c4 mail['reply-to'] #=> '#<Mail::Field:0x180e5e8 @field=#<Mail::ReplyToField:0x180e1c4 mail['Reply-To'] #=> '#<Mail::Field:0x180e5e8 @field=#<Mail::ReplyToField:0x180e1c4 mail[:reply_to].encoded #=> 'Reply-To: Mikel Lindsaar <mikel@test.lindsaar.net>, ada@test.lindsaar.net\r\n' mail[:reply_to].decoded #=> 'Mikel Lindsaar <mikel@test.lindsaar.net>, ada@test.lindsaar.net' mail[:reply_to].addresses #=> ['mikel@test.lindsaar.net', 'ada@test.lindsaar.net'] mail[:reply_to].formatted #=> ['Mikel Lindsaar <mikel@test.lindsaar.net>', 'ada@test.lindsaar.net']