Seems that next Symfony release will break this library because symfony/http-foundation 5.1.0-RC1 doesn't depend on symfony/mime anymore, and this library still uses it's abstractions here.

There are two ways to solve the problem:

• Tag this library as conflicting with symfony/http-foundation 5.1+.
• Make new release that drops support for ancient PHP versions by requiring symfony/mime explicitly.

I vote for the second way. Composer will preserve legacy systems supporters from installing new versions of the library, so it won't break their tests.

The third way is a combination of both: setting conflict in a minor release and then making a major one that uses symfony/mime instead of symfony/http-foundation.