Problem with headers encoding in sent e-mail?

It seems a bug to me.
My web server runs drupal 5.1 and subscriptions module 5.x-1.x-dev, but the same happens with 5.x-1.4

Notification e-mails are generated with weird From:, Reply-To: and Errors-To: fields.
Read below (copied/pasted from source, replaced actual domain with example.it):

Errors-To: =?UTF-8?B?RnJvbTogYXNzaXN0ZW56YS13ZWJAbmV3Y29tc3JsLml0ClJlcGx5LXRvOiBhc3M=?=@example.it,
=?UTF-8?B?aXN0ZW56YS13ZWJAbmV3Y29tc3JsLml0ClgtTWFpbGVyOiBEcnVwYWwKUmV0dXI=?=@example.it,
=?UTF-8?B?bi1wYXRoOiBhc3Npc3RlbnphLXdlYkBuZXdjb21zcmwuaXQKRXJyb3JzLXRvOiA=?=@example.it,
=?UTF-8?B?YXNzaXN0ZW56YS13ZWJAbmV3Y29tc3JsLml0?=@guardian.newcomsrl.it
Reply-To: =?UTF-8?B?RnJvbTogYXNzaXN0ZW56YS13ZWJAbmV3Y29tc3JsLml0ClJlcGx5LXRvOiBhc3M=?=@example.it,
=?UTF-8?B?aXN0ZW56YS13ZWJAbmV3Y29tc3JsLml0ClgtTWFpbGVyOiBEcnVwYWwKUmV0dXI=?=@example.it,
=?UTF-8?B?bi1wYXRoOiBhc3Npc3RlbnphLXdlYkBuZXdjb21zcmwuaXQKRXJyb3JzLXRvOiA=?=@example.it,
=?UTF-8?B?YXNzaXN0ZW56YS13ZWJAbmV3Y29tc3JsLml0?=@example.it
From: =?UTF-8?B?RnJvbTogYXNzaXN0ZW56YS13ZWJAbmV3Y29tc3JsLml0ClJlcGx5LXRvOiBhc3M=?=@example.it,
=?UTF-8?B?aXN0ZW56YS13ZWJAbmV3Y29tc3JsLml0ClgtTWFpbGVyOiBEcnVwYWwKUmV0dXI=?=@example.it,
=?UTF-8?B?bi1wYXRoOiBhc3Npc3RlbnphLXdlYkBuZXdjb21zcmwuaXQKRXJyb3JzLXRvOiA=?=@example.it,
=?UTF-8?B?YXNzaXN0ZW56YS13ZWJAbmV3Y29tc3JsLml0?=@example.it

E-mail clients cannot parse properly such headers, and show almost non-sense content in header data (e.g., From: field is invalid, a very long string with fragments of random header strings).

The body part, instead, is fine.

This is a serious issue, to me.

Hope you can help me fix the problem.
--
Gabriele