"that you not be quickly shaken from your composure or be disturbed either by a spirit or a message or a Letter as if from us, to the effect that the day of the Lord has come." (2 Thessalonians 2:2)
The above reference shows us that even the earliest Christians were aware of the possibility of pseudonymous letters, which bolsters our confidence in having the full New Testament canon. It is also worth noting that Paul wrote his signature at the end of each epistle circulated by him (1 Corinthians 16:21; 2 Thessalonians 3:17; Colossians 4:18; Galatians 6:11; Philemon 19).
