Encode a line break in an HTML email sent by Rules?
Have a look at my answer to "How to use Rules to send an eMail with a body containing all items of a field collection?". It includes a rule (in export format) which looks like so:

{ "rules_field_collection_msg_and_email_body" : {
    "LABEL" : "Field Collection msg and eMail body",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "ON" : { "node_view" : [] },
    "IF" : [
      { "entity_has_field" : { "entity" : [ "node" ], "field" : "field_article_details" } },
      { "user_has_role" : {
          "account" : [ "site:current-user" ],
          "roles" : { "value" : { "3" : "3" } }
        }
      }
    ],
    "DO" : [
      { "variable_add" : {
          "USING" : {
            "type" : "text",
            "value" : "\u003Ch2\u003EThis is a demo of a node containing a field collection ... \u003C\/h2\u003E\r\n\u003Chr \/\u003E\r\n\u003Cstrong\u003EAbout this demo\u003C\/strong\u003E:\u003Cbr \/\u003E\r\n\u003Cbr \/\u003E\r\nThe machine name of the field collection is \u003Cstrong\u003Efield_article_details\u003C\/strong\u003E, and the number of values for it is unlimited.\u003Cbr \/\u003E\r\nThis field collection has these fields:\u003Cbr \/\u003E\u003Cbr \/\u003E\r\n- a text field labeled \u003Cstrong\u003EAuthor\u003C\/strong\u003E with machine name \u003Cstrong\u003Efield_author\u003C\/strong\u003E.\u003Cbr \/\u003E\r\n- a decimal field labeled \u003Cstrong\u003EPrice\u003C\/strong\u003E with machine name \u003Cstrong\u003Efield_price\u003C\/strong\u003E.\u003Cbr \/\u003E\r\n- a boolean field labeled \u003Cstrong\u003EApproval required\u003C\/strong\u003E with machine name \u003Cstrong\u003Efield_approval_required\u003C\/strong\u003E.\u003Cbr \/\u003E\u003Cbr \/\u003E\r\n\u003Chr \/\u003E\r\n\u003Cstrong\u003EPart 1\u003C\/strong\u003E: The  list of items contained in the field collection with machine name \u003Cstrong\u003E\u0027field_article_details\u0027\u003C\/strong\u003E looks like so (note the comma to separate each item):\r\n\u003Chr \/\u003E\r\n[node:field_article_details]\r\n \u003Chr \/\u003E\r\n\u003Cstrong\u003EPart 2\u003C\/strong\u003E: The result of a \u003Cstrong\u003ERules loop\u003C\/strong\u003E that iterates for each of the field collection items (using its unique ID) ...\r\n\u003Chr \/\u003E"
          },
          "PROVIDE" : { "variable_added" : { "variable_for_email_body" : "Variable to use in eMail body" } }
        }
      },
      { "drupal_message" : { "message" : "\u003Ch2\u003EThis is a demo of a node containing a field collection ... \u003C\/h2\u003E\r\n\u003Chr \/\u003E\r\n\u003Cstrong\u003EAbout this demo\u003C\/strong\u003E:\u003Cbr \/\u003E\r\n\u003Cbr \/\u003E\r\nThe machine name of the field collection is \u003Cstrong\u003Efield_article_details\u003C\/strong\u003E, and the number of values for it is unlimited.\u003Cbr \/\u003E\r\nThis field collection has these fields:\u003Cbr \/\u003E\u003Cbr \/\u003E\r\n- a text field labeled \u003Cstrong\u003EAuthor\u003C\/strong\u003E with machine name \u003Cstrong\u003Efield_author\u003C\/strong\u003E.\u003Cbr \/\u003E\r\n- a decimal field labeled \u003Cstrong\u003EPrice\u003C\/strong\u003E with machine name \u003Cstrong\u003Efield_price\u003C\/strong\u003E.\u003Cbr \/\u003E\r\n- a boolean field labeled \u003Cstrong\u003EApproval required\u003C\/strong\u003E with machine name \u003Cstrong\u003Efield_approval_required\u003C\/strong\u003E.\u003Cbr \/\u003E\u003Cbr \/\u003E\r\n\u003Chr \/\u003E\r\n\u003Cstrong\u003EPart 1\u003C\/strong\u003E: The list of items contained in the field collection with machine name \u003Cstrong\u003E\u0027field_article_details\u0027\u003C\/strong\u003E looks like so (note the comma to separate each item):\r\n\u003Chr \/\u003E\r\n[node:field_article_details]\r\n \u003Chr \/\u003E\r\n\u003Cstrong\u003EPart 2\u003C\/strong\u003E: The result of a \u003Cstrong\u003ERules loop\u003C\/strong\u003E that iterates for each of the field collection items (using its unique ID) ...\r\n\u003Chr \/\u003E" } },
      { "LOOP" : {
          "USING" : { "list" : [ "node:field-article-details" ] },
          "ITEM" : { "article_details_item" : "article details item" },
          "DO" : [
            { "data_set" : {
                "data" : [ "variable-for-email-body" ],
                "value" : "[variable-for-email-body:value]\u003Cbr \/\u003E\r\nThe host entity of this field collection item: [article-details-item:host]\u003Cbr \/\u003E\r\nThe unique ID of the field collection item: [article-details-item:item-id]\u003Cbr \/\u003E\r\nField \u0022field_author\u0022: [article-details-item:field-author]\u003Cbr \/\u003E\r\nField \u0022field_price\u0022: [article-details-item:field-price]\u003Cbr \/\u003E\r\nField \u0022field_approval_required\u0022: [article-details-item:field-approval-required]\u003Cbr \/\u003E\r\n\u003Chr \/\u003E"
              }
            },
            { "drupal_message" : { "message" : "The host entity of this field collection item: [article-details-item:host]\u003Cbr \/\u003E\r\nThe unique ID of the field collection item: [article-details-item:item-id]\u003Cbr \/\u003E\r\nField \u0022field_author\u0022: [article-details-item:field-author]\u003Cbr \/\u003E\r\nField \u0022field_price\u0022: [article-details-item:field-price]\u003Cbr \/\u003E\r\nField \u0022field_approval_required\u0022: [article-details-item:field-approval-required]\u003Cbr \/\u003E\r\n\u003Chr \/\u003E" } }
          ]
        }
      },
      { "mail" : {
          "to" : [ "site:current-user:mail" ],
          "subject" : "Demo of an eMail with a body containing all items of a field collection",
          "message" : [ "variable-for-email-body" ],
          "language" : [ "" ]
        }
      }
    ]
  }
}


Even though your question is not a duplicate of that question, this rule includes a solution which you could also apply in your case:


A Rules Action Add a variable, with machine name variable_for_email_body.
A Rules Action to actually populate the content of this variable_for_email_body (just replace that content to fit your needs, and include whatever HTML tags you want to use in our eMail body).
A Rules Action Send mail, with Message body equal to ... [variable-for-email-body].


Done ...

PS: obviously, this is for D7 ...