Messages to gmail bouncing

[#1]

I'm having problems sending email to a gmail address today. The address is apparently valid (checks out OK using http://www.validmail.org/) but I keep getting delivery failure notifications:
-------------
The following recipient(s) could not be reached:
mycontact@gmail.com.
  Error Type: SMTP
  Remote server (212.159.8.107) issued an error.
  hMailServer sent: RCPT TO:<mycontact@gmail.com.>
  Remote server replied: 550 <mycontact@gmail.com.> invalid address
-------------
My setup is configured to send via relay.plus.net. Nothing has changed recently, and I'm not having problems sending other mails (including a test to my own gmail account, which came through just fine).
Any suggestions as to what could be going wrong, or how to narrow it down?

[#2]

Hi Boxersoft,
We're not aware of any changes on our side that should be causing this issue so I'm a bit miffed 
A few questions that may help us narrow this down are:
1. Is the mail being sent from a machine or device connected to you Plusnet connection?
2. Has there been any update to mail apps or clients you may be using that could have caused settings to change?
3. have you logged into and sent test emails to/from each of the accounts to check all is working?

[#3]

> 1. Is the mail being sent from a machine or device connected to you Plusnet connection?
Yes.
2. Has there been any update to mail apps or clients you may be using that could have caused settings to change?
No.
3. have you logged into and sent test emails to/from each of the accounts to check all is working?
Yes - as I said, I'm able to send mails to other addresses without problems. The recipient email address is not one that I have sent to before today, so my first thought was that it was invalid. The ValidMail service seems to check it out OK though, and I got a friend (who uses an ISP other than PlusNet) to send a test mail - apparently successfully.
I might have assumed a DNS problem but since I'm able to send to other addresses @gmail.com that doesn't seem likely, and I simply don't know what else to check.

[#4]

I know that you have edited the address in the message but it shows

Quote

<mycontact@gmail.com.>

Note the dot after the com
If this is present in the address you are using it would cause this error message.

[#5]

Oh, bugger - as simple as that? The mail address was passed on to me by someone else and I didn't notice that trailing dot coming over. Thanks very much indeed for pointing that out.
<shuffles off, hanging head in shame...>