Its the other way around actually: ActionMailer 4.2 depends on mail ~&gt; 2.5, &gt;= 2.5.4, so you&#x27;ll get 2.6.3 now. That version depends on mime-types &lt;3, &gt;= 1.16, so you&#x27;ll get 2.99.<p>The big change in mime-types 3 is using the columnar store by default, which is where the memory savings come from. It&#x27;s opt-in from mime-types 2.6 onwards because it&#x27;s a breaking change. Mail and afaik most other gems have opted in already.