diff --git a/locales/en.json b/locales/en.json
index e7bfd2ded..37160d403 100644
--- a/locales/en.json
+++ b/locales/en.json
@@ -235,7 +235,7 @@
     "migration_0003_not_jessie": "The current debian distribution is not Jessie !",
     "migration_0003_system_not_fully_up_to_date": "Your system is not fully up to date. Please perform a regular upgrade before running the migration to stretch.",
     "migration_0003_still_on_jessie_after_main_upgrade": "Something wrong happened during the main upgrade : system is still on Jessie !? To investigate the issue, please look at {log} :s ...",
-    "migration_0003_general_warning": "Please note that this migration is a delicate operation. While the YunoHost team did its best to review and test it, the migration might still break parts of the system or apps.\n\nTherefore, we recommend you to :\n    - Perform a backup of any critical data or app ;\n    - Be patient after launching the migration : depending on your internet connection and hardware, it might take up to a few hours for everything to upgrade.",
+    "migration_0003_general_warning": "Please note that this migration is a delicate operation. While the YunoHost team did its best to review and test it, the migration might still break parts of the system or apps.\n\nTherefore, we recommend you to :\n    - Perform a backup of any critical data or app. More infos on https://yunohost.org/backup ;\n    - Be patient after launching the migration : depending on your internet connection and hardware, it might take up to a few hours for everything to upgrade.\n\nAdditionally, the port for SMTP, used by external email clients like (Thunderbird or K9-Mail) was changed from 465 (SSL/TLS) to 587 (STARTTLS). The old port 465 will automatically be closed and the new port 587 will be opened in the firewall. You and your users *will* have to adapt the configuration of your email clients accordingly!",
     "migration_0003_problematic_apps_warning": "Please note that the following possibly problematic installed apps were detected. It looks like those were not installed from an applist or are not flagged as 'working'. Consequently, we cannot guarantee that they will still work after the upgrade : {problematic_apps}",
     "migration_0003_modified_files": "Please note that the following files were found to be manually modified and might be overwritten at the end of the upgrade : {manually_modified_files}",
     "migrations_backward": "Migrating backward.",
diff --git a/src/yunohost/data_migrations/0003_migrate_to_stretch.py b/src/yunohost/data_migrations/0003_migrate_to_stretch.py
index 8b8d37447..6900b678a 100644
--- a/src/yunohost/data_migrations/0003_migrate_to_stretch.py
+++ b/src/yunohost/data_migrations/0003_migrate_to_stretch.py
@@ -15,6 +15,7 @@ from yunohost.service import (_run_service_command,
                               manually_modified_files_compared_to_debian_default)
 from yunohost.utils.filesystem import free_space_in_directory
 from yunohost.utils.packages import get_installed_version
+from yunohost.firewall import firewall_allow, firewall_disallow
 
 logger = getActionLogger('yunohost.migration')
 
@@ -72,6 +73,11 @@ class MyMigration(Migration):
         os.system("apt autoremove --assume-yes")
         os.system("apt clean --assume-yes")
 
+        # We moved to port 587 for SMTP
+        # https://busylog.net/smtp-tls-ssl-25-465-587/
+        firewall_allow("Both", 587)
+        firewall_disallow("Both", 465)
+
         # Upgrade yunohost packages
         logger.warning(m18n.n("migration_0003_yunohost_upgrade"))
         self.restore_files_to_keep()