MTA stops, wont restart without full restart

[bctechnzl]

During our offsite backup procedure, we do a hot rsync backup of the /opt/zimbra directory. Then we zmcontrol stop, run a cold sync and zmcontrol start- all within the backup script
Occasionally, and as happened this morning, the mta and logger failed to start. Screeds of reporting data flows into the zimbra log, all from zimbramon, for example:

<Code insert>
zimbramon[28632]: :::2BA6A070-AA0A-11DF-A54E-AF92BD8351AB:::35650637,3,0,0,0,0,0,0,0,0,0,0,2799 4,0,0,0,1,0,0,1518,0,0,0,0,0,1,0,0,0,0,0,0,0,0,595 ,0,0,0,0,0,0,0,0,0,0,0,0,0,1144,0,0,0
,0,0,36790,0,36291,110354,0,0,0,4,0,0,0,0,0,0,0,14 38,0,0,0,0,0,0,0,0,0,0,4601,0,1,0,4,0,0,0,0,0,0,0, 0,0,0,0,4576,57,0,0,0,0,0,0,OFF,4606,82,33,3053,0, 0,0,1,35825,0,0,0,90,1325142,4629037,37264,131445, 718278
,7032,0,0,0,716855,7111,0,26927,1,120,7232,35,17,1 5338601,7679,0,59174,17148,0,0,0,149917696,8369,32 681,895520768,26927,3195,0,21653,4064,6423,0,0,119 68512,16384,45,9017,26927,0,0,0,0,0,311,832,526068 2,5524
,0,7245,0,0,0,0,0,0.000000,4,0,51,0,149,156,0,0,0, 0,0,0,0,0,0,228247,228247,NULL,0,0,11337,0,2924,0, 0,OFF,0,37,14,814,28266,52,0,0,0,,,0,0,0,0,0,0,0,0 ,0,NONE,0,0,0,0,0,0,0,,44139,0,0,0,0,1,3,4,1,86418

</Code insert>

Users experience the lack of ability to send emails, but the rest of the interface appears unaffected. I had to manually stop and start zimbra to get it to work again, a zmmtactl restart was insufficient.

I'm not sure if this is related to the power failure shutdown we had on Saturday, which I called in a support ticket on and subsequently closed as the server became operational again.

Suggestions appreciated