Send emails using Erlang gen_smtp shortcut.

email.erl

% Send plaintext email using gen_smtp https://github.com/Vagabond/gen_smtp
% This function sends email directly to receiver's SMTP server and don't use MTA relays.
%
% Example plaintext email:
% Mail = mail_plain(<<"Bob <sender@example.com>">>, <<"Alice <receiver@example.com>">>, <<"The mail subject">>, <<"The mail body">>),
% send_email(Mail).
%
% Example email with image attachment:
% ImgName = "image.jpg",
% {ok, ImgBin} = file:read_file(ImgName),
% Mail = mail_with_attachments(<<"sender@example.com">>, <<"receiver@example.com">>, <<"Photos">>,
%                              <<"See photo in attachment">>,
%                              [{ImgName, "image/jpeg", ImgBin}]),
% send_email(Mail).

-spec send_email(email()) -> binary() | {error, atom(), any()} | {error, any()}.
send_email(Email) ->
    {From, [To], _Body} = Email,
    [_, FromDomain] = split_addr(From),
    [_, ToDomain] = split_addr(To),
    Options = [{relay, binary_to_list(ToDomain)},
               {tls, if_availablex},
               {hostname, FromDomain}],
    gen_smtp_client:send_blocking(Email, Options).

-spec mail_plain(binary(), binary(), binary(), binary(), Opts) -> email() when
      Opts :: [{headers, [{binary(), binary()}]}].
mail_plain(From, To, Subject, Body, Opts) ->
    AddHeaders = proplists:get_value(headers, Opts, []),
    Mimemail =
        {<<"text">>, <<"plain">>,
         [
          {<<"From">>, From},
          {<<"To">>, To},
          {<<"Subject">>, Subject},
          {<<"Content-Type">>, <<"text/plain; charset=utf-8">>}
          | AddHeaders],
         [{<<"transfer-encoding">>, <<"base64">>}],
         Body},
    FromAddr = extract_addr_rfc822(From),
    {FromAddr, [extract_addr_rfc822(To)], mimemail:encode(Mimemail)}.

-spec mail_with_attachments(
        binary(), binary(), binary(), binary(),
        [{Name :: binary(), MimeType :: binary(), Body :: binary()}]) -> email().
mail_with_attachments(From, To, Subject, Body, Attachments) ->
    MimeBody = {<<"text">>, <<"plain">>,
                [{<<"Content-Type">>, <<"text/plain;charset=utf-8">>},
                 {<<"Content-Transfer-Encoding">>, <<"quoted-printable">>},
                 {<<"Content-Disposition">>, <<"inline">>}],
                [],
                Body},
    MimeAttachments = [begin
                           [Ct1, Ct2] = binary:split(MimeType, <<"/">>),
                           {Ct1, Ct2,
                            [{<<"Content-Transfer-Encoding">>, <<"base64">>}],
                            [{<<"disposition">>, <<"attachment">>},
                             {<<"disposition-params">>,
                              [{<<"filename">>, Name}]}],
                            AtBody}
                       end
                       || {Name, MimeType, AtBody} <- Attachments],
    Mimemail = {<<"multipart">>,
               <<"mixed">>,
               [{<<"From">>, From},
                {<<"To">>, To},
                {<<"Subject">>, Subject}],
               [],
               [MimeBody | MimeAttachments]},
    FromAddr = extract_addr_rfc822(From),
    {FromAddr, [extract_addr_rfc822(To)], mimemail:encode(Mimemail)}.


extract_addr_rfc822(Rfc822) ->
    {ok, [{_, Addr}]} = smtp_util:parse_rfc822_addresses(Rfc822),
    list_to_binary(Addr).

split_addr(MailAddr) ->
    binary:split(MailAddr, <<"@">>).