I think it should change to: Promise<string | undefined>

[FarzanKh]

modern-full-stack-development/mailbag/server/src/IMAP.ts

Line 172 in 270a829

public async getMessageBody(inCallOptions: ICallOptions): Promise<string> {

[TomFrink]

Due to this breaking change in Typescript
I switched this in Contacts:
public deleteContact(inID: string): Promise<string | void>
And this in SMTP:
public sendMessage(inOptions: SendMailOptions): Promise<string | void> {