Avoid unpredictable behaviour on weird `undisclosed recipients:;` signal

IMAP does not return the bcc field cleanly, in fact, node-imap and by extension imap-simple wont return a valid bcc (or cc for that matter) at all.
Generally, you can only view those with the "envelope" attribute of a mail, but those are empty. Only clean option would be to somehow accesss and work with the rfc822 spec of `Original-Recipient`.

application/imap-service.js

@@ -347,6 +347,7 @@ class ImapService extends EventEmitter {
 
 	async _getMailHeaders(uids) {
 		const fetchOptions = {
+			envelope: true,
 			bodies: ['HEADER.FIELDS (FROM TO SUBJECT DATE)'],
 			struct: false
 		}

domain/mail-repository.js

@@ -27,7 +27,9 @@ class MailRepository {
 	}
 
 	add(to, mailSummary) {
-		this.mailSummaries.set(to.toLowerCase(), mailSummary)
+		if (to !== 'undefined') {
+			this.mailSummaries.set(to.toLowerCase(), mailSummary)
+		}
 	}
 
 	removeUid(uid, address) {