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[iip]

One note about the mailserver check:
I can input addresses of the type mail+abc@domain.com but you send the mail to "mail abc@domain.com" (the plus sign seems to be filtered out) and is therefore rejected by the mailserver.

[kriteknetworks]

Maybe create a temp account without + in it and retry the test?

[iip]

+ is indeed not part of any email address but can be used as an extension, i.e. if you have the address
email@domain.com you can use email+spam@domain.com email+xxx@domain.com email+whatever@domain.com and they will/should all arrive at email@domain.com, without any configuration. I use it for example to find out who sells my email address.
What I wanted to say was, either you should support +es in the email address and transmit them accordingly or just tell the user that the email address he entered was not valid. Anyway it should be valid as the use of the + sign is allowed and valid:
http://tools.ietf.org/html/rfc3696#section-3
http://tools.ietf.org/html/rfc5322#section-3.4.1

[snarked]

Although your conclusion is correct, I disagree with your statement "+ is indeed not part of any email address...."  Every host is free to decide for itself what is and isn't valid in the LOCAL-PART of a mailbox address.

[iip]

Yeah, alright, the recipient delimiter need not be a plus sign.
However all of the characters from the RFC should be allowed to be used in the local part.