feat: send mailings over MailJet smtp in dedicated worker process

.env

@@ -38,6 +38,9 @@ MESSENGER_TRANSPORT_DSN=doctrine://default?auto_setup=0
 
 ###> symfony/mailer ###
 MAILER_DSN=null://null
+# Has to resolve in every environment or the container does not compile, see the
+# mailjet section below for the production value
+MAILING_MAILER_DSN=null://null
 ###< symfony/mailer ###
 
 APP_BASE_URI=https://myep-team.ddev.site
@@ -68,8 +71,18 @@ MYEP_OAUTH2_URL_RESOURCE_OWNER_DETAILS=https://my.ep-reisen.de/api/userinfo
 MYEP_OAUTH2_SCOPES=email,id,roles,profile
 
 ###> symfony/mailjet-mailer ###
-# MAILER_DSN=mailjet+api://PUBLIC_KEY:[email protected]
-# #MAILER_DSN=mailjet+smtp://PUBLIC_KEY:[email protected]
+# Bulk teamer mailings only, everything else goes out over MAILER_DSN. Set the real
+# credentials in .env.local - the sending domain has to be verified in Mailjet (SPF,
+# DKIM, DMARC) or the mail is delivered but lands in spam.
+#
+# Use smtp. The api transport does not send the message we built: it takes the parts as
+# json fields and has Mailjet assemble the mail, which drops the embedded logo into an
+# ordinary attachment. Over smtp our own MIME goes out unchanged and the mail arrives as
+# it was rendered. Deliverability is identical either way, it is the same account, the
+# same ip pool and the same dkim signature. The api is only worth revisiting if the host
+# ever blocks outbound port 587, since it talks https instead.
+# MAILING_MAILER_DSN=mailjet+smtp://PUBLIC_KEY:[email protected]
+# MAILING_MAILER_DSN=mailjet+api://PUBLIC_KEY:PRIVATE_KEY@default
 ###< symfony/mailjet-mailer ###
 
 BIN_GS=/usr/bin/gs