Building plain text emails with an .erb template doesn't allow you to indent code like you normally do in HTML mails.

DON'T

<%= 'foo' if bar %>

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"\n" if bar is false

"foo\n" if bar is true

<%= nil %>

"\n"

<% if true %>
  <%= 'foo' %>
<% end %>  

" foo"

<%= 'foo' %>

<%= 'bar' %>

"foo\n\nbar\n"

DO

Write unindented code to get the expected result.

<% if bar %>
<%= 'bar' %>
<% end %>
<%= 'foo' %>
<%= 'bar' %> 

• Use Form Models Show archive.org snapshot to move logic out of the template.
• Sometimes it makes sense to use .haml. It helps to write more beautiful code, but makes indentation more difficult.

Understanding trim mode for .erb

.erb has two approaches to handle leading spaces (<%- %>) and trailing newlines (<% -%>).

ERB.new("  <% %>foo").result => "  foo"
ERB.new("  <%- %>foo", nil, '-').result => "foo"

ERB.new("<% %>\nfoo\n").result => "\nfoo\n"
ERB.new("<% -%>\nfoo\n", nil, '-').result => "foo\n"

• You can combine removing leading spaces and removing trailing newlines.
• You need to enable trim mode for erb like above: new(str, safe_level=nil, trim_mode=nil, eoutvar='_erbout')

Difference between Rails mailer and ERB

Rails mailer

<% if true %>
<%= 'bar' %>
<% end %>
<%= 'foo' %>
<%= 'bar' %>

bar
foo
bar

ERB

<% if true %>
<%= 'bar' %>
<% end %>
<%= 'foo' %>
<%= 'bar' %>

bar

foo
bar