feat: add "null" SMTP transport mode #14184

Job	Run time
changes	7s
integration-sqlite (capabilities_features, 8.1, main)	1m 36s
integration-sqlite (collaboration_features, 8.1, main)	2m 4s
integration-sqlite (comments_features, 8.1, main)	2m 38s
integration-sqlite (dav_features, 8.1, main)	5m 6s
integration-sqlite (features, 8.1, main)	6m 13s
integration-sqlite (federation_features, 8.1, main)	4m 16s
integration-sqlite (--tags ~@large files_features, 8.1, main)	11m 50s
integration-sqlite (filesdrop_features, 8.1, main)	1m 51s
integration-sqlite (openldap_features, 8.1, main)	1m 41s
integration-sqlite (openldap_numerical_features, 8.1, main)	1m 39s
integration-sqlite (ldap_features, 8.1, main)	2m 0s
integration-sqlite (remoteapi_features, 8.1, main)	1m 31s
integration-sqlite (setup_features, 8.1, main)	1m 18s
integration-sqlite (sharees_features, 8.1, main)	4m 18s
integration-sqlite (sharing_features, 8.1, main)	13m 43s
integration-sqlite (videoverification_features, 8.1, main)	5m 1s
integration-sqlite-summary	3s
	1h 6m 55s

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