Undeliverable email - The following recipient(s) cannot be reached - Server error '550 5.5.0

[MikeT62]

Hi Folks,

Any suggestions why I'm unable to send emails and getting this back from System Administrator?

I have an email I send out to 20 friends every couple of months and yesterday I received this Undeliverable message.

So I tried sending them individually and each one was blocked?

Any clues?

Thank you

[Gondola]

Hi Mike

Usually the first error message will give an address that has caused the failure notification.

The fastest way to identify multiple failures is to split a distribution list into smaller lists. For example two groups of 10. Then any fail into two groups of 5. You may find the smaller group sends without failing and that would tell you that all addresses in that group are good.

[MikeT62]

Thank you very much. Now I understand. One unavailable address blocks sending to all, so if I try again then the system assumes I’m spamming! I’ll be patient, wait 24 hours and send separate messages to find out which one is now unavailable. 

[Gondola · Accepted solution]

Hi Mike

Your distribution list clearly includes some addresses that are not available for delivery. So you had a permanent fail bounce back message.

If you retry sending to unavailable addresses the system response is to block your sending. It's a standard response intended to put a temporary stop to spammers trying to guess email addresses.

Normally the stop on sending will be relaxed after an hour but may still rate limit sending for up to 24 hours. So you may not be able to resume group sending until the rate limiting is fully cleared.

You should ensure the distribution list has unavailable addresses removed or you'll find rate limiting or a stop on sending will happen again.