What is the "msv1.invalid" domain and why am I having trouble delivering email?

The msp1.invalid domain is NOT REAL.  It was first implemented at outlook.com around 2013 as a "placeholder" and many organizations left it in their DNS when they should remove it.

From Microsoft:

The .msv1.invalid MX is intentional since it’s a placeholder used only for ownership checks, but some DNS hosts wrongly block it. You do not need that MX to verify the domain, TXT-only verification is supported and is the recommended path if your DNS UI rejects .invalid.

If you dig a major insurance company their MX tree might show this:

insusomething.com.1383INMX5     mx0a-001bde01.pphosted.com.
insusomething.com.1383INMX32767 ms46086885.msv1.invalid.     <-- invalid
insusomething.com.1383INMX32767 ms29530564.msv1.invalid.     <-- invalid
insusomething.com.1383INMX5     mx0b-001bde01.pphosted.com.

So when you send email to their domain, hosted by pphosted.com, if it can't be delivered or is deferred and your system keeps trying the MX chain in order, it will eventually hit the "...msv1.invalid." MX records and of course they cannot be connected to as the domain is invalid at this time.

These records should be removed.

Here are others you might see:

ms29530564.msv1.invalid
ms46086885.msv1.invalid
ms29530564.msv1.invalid
ms29530564.msv1.invalid
ms29530564.msv1.invalid
ms46086885.msv1.invalid
ms29530564.msv1.invalid
ms29530564.msv1.invalid
ms46086885.msv1.invalid
ms46086885.msv1.invalid

All invalid.