Following Tuesday night's 1-0 victory over Rotherham United, West Ham had moved to within just one spot of the third-placed Blues who had lost 2-3 at Stoke City.
Not surprisingly, Alan Pardew named an unchanged side while Joe Royle - still smarting over last season's play-off semi-final defeat against at the hands of the Hammers - made just one crucial switch as Counago replaced Shefki Kuqi.
But after the events of last May, the Suffolk side quickly found yet more salt being rubbed into their East End wounds. And to make matters worse the damage was inflicted by a former Norwich City player.
For when Town only half-cleared an 11th-minute corner, Nigel Reo-Coker played in the overlapping Luke Chadwick and his pinpoint centre was headed home by Mackay who claimed his first goal since his £300,000 move from Carrow Road just eight days ago.
In reply, Darren Bent - who had already seen his early shot cannon off Stephen Bywater's legs - sent another effort just a whisker wide.
But buoyed by that goal from their former Canary, West Ham were well and truly on song.
Marlon Harewood flashed a shot inches over the angle, and after Sheringham had headed narrowly wide, Reo-Coker then saw his low 20-yarder acrobatically palmed aside by Kelvin Davis.
As the half-hour mark passed, the Ipswich keeper was equally defiant as he produced a simply sensational double save to deny goalscorer Mackay and then the dumbstruck Sheringham, who shook his adversary's hand in salute.
On 33 minutes, the rivalry looked set to be tested once more when the Ipswich keeper upended the fleeing Carl Fletcher, but this time the former England striker failed to extend the relieved Davis as the consequent penalty flew past the base of his right-hand post and into the boards.
Sheringham's costly miss proved a right Royle escape for the Blues who continued to persevere with their passing plan, and just before the interval, only Bywater's fingertips prevented Tommy Miller from sending the sides in level.
And ten minutes after the break, the Hammers' keeper was again called upon to rescue his side when he denied Miller from point-blank range with his knees. But Bywater's relief was short-lived as Miller dusted himself down and whipped over the resultant corner which Counago bundled home to claim the equaliser.
And seeing their victory ebbing away, West Ham finished the game grateful that they had still claimed a point after the breaking Tractor Boys saw Bent climax their 65th-minute trundle upfield with a sizzling 18-yarder that crashed off the bar.