Lisa

[Unregistered]

What is considered a "formal name" when addressing envelopes, invitations, etc.?

[Ouisch]

Use the addressee's full given name and any appropriate titles, not a nickname:

Ms. Margaret Smith instead of "Peggy Smith"
Mr. Charles Winchester III instead of "Chuck Winchester"

[lmcopeland]

Always use the Mr. and Ms. or Mrs. when addressing envelopes?

[Ouisch]

Always use the Mr. and Ms. or Mrs. when addressing envelopes?

Yes.